By-election winner Lindsay Whittle has been sworn in at the Welsh Parliament, meaning Plaid Cymru now officially represents Caerphilly
He earlier said he would "work like a Trojan" after ousting Labour from the seat for the first time in a century
Plaid leader Rhun ap Iorwerth says the victory is "a reset for Welsh politics" - here's the wider political reaction
Welsh First Minister Eluned Morgan admits this was a "bad result for Labour" - but says she will not resign
Reform candidate Llyr Powell - who came second - says he's confident his party will win enough seats at next year's Welsh election
The prospect for Labour of heavy defeat across the country in May looks more likely than ever, writes our political editor
Meanwhile, we've been hearing from voters in Caerphilly, who say it is "time for Wales to stand up for itself" and "Labour have been complacent"
Lindsay Whittle sworn in at Welsh Parliament after historic Caerphilly by-election win
Oliver Slow
BBC Wales
It’s just gone midnight at the Caerphilly Leisure Centre, where counting is under way.
Here are some pictures from the count.
Image source, Matthew Horwood
Image source, Mark Lewis/BBC
Image source, Matthew Horwood
Gareth Lewis
BBC Wales political editor
Image source, Matthew HorwoodReform officials say it will be "very close" as the count continues in Caerphilly
Across the hall from Plaid's Lindsay Whittle a group of Reform officials have gathered.
There are tense faces and furrowed brows.
I ask them how they think it is going and one of them tells me he needs to look at the data.
No one answers directly when I ask if they think Plaid might have edged it.
"Very close," says one, repeating tonight's mantra.
David Deans
BBC Wales political reporter
Lindsay Whittle is looking like a man that thinks he might win this. Plaid Cymru's candidate told me he was feeling "buoyant, and we're doing well".
"We've looked at the boxes so far and we're encouraged," he said.
"It's close, yes its very close." He said Labour's vote had "totally disappeared".
He was hesitant to go further, but he added: "It's looking very positive."
Image source, Mark Lewis/ BBCOliver Slow
BBC Wales
Labour has held Caerphilly ever since devolution began in 1999.
As the below graphic shows, the party has won all six Senedd elections since then, followed by Plaid Cymru.
With multiple Labour sources already admitting defeat, the party’s stronghold on the constituency looks like it could be about to end.
Gareth Lewis
BBC Wales political editor
Image source, Matthew HorwoodThere are some nervous faces as the count continues
Two words keep cropping up here: "Very close." You can almost feel the nerves.
It does not matter if you speak to Plaid Cymru or Reform, they both say the same.
"We've fought for every vote" has also cropped up, and I do not get the impression that party representatives are being coy.
They genuinely do not know if they have done enough to win.
A question for Reform has been will they get their voters out? Will they manage to do it here in Caerphilly?
Has Plaid wooed enough wavering or disillusioned Labour voters?
There is no dispute from anyone here that Labour has lost.
David Deans
BBC Wales political reporter
Just had a chat with a Tory source.
"I don't know about you but I don't think we're going to win this," they joked, having found no Conservative votes in the bit of the constituency they were watching votes come in from.
The source said they heard people "doing weird things" - giving the example of at least one Tory voter backing Plaid Cymru to stop Reform (as well as other Tory voters supporting Reform).
That is not something you would usually expect given Plaid Cymru is pro-independence and the Conservatives are a unionist party.
Oscar Edwards
BBC Wales
Image source, Matthew HorwoodZia Yusuf is Reform UK's head of policy
Reform UK's head of policy Zia Yusuf, who is at the Caerphilly by-election, has said it is a "historic" day for Wales.
Speaking to GB News he said it is a "big moment" for the Reform party as it battles Plaid for control of the constituency.
"The Labour Party and the Tory Party, the two old great parties that have dominated UK politics, will struggle to get even a quarter of the vote share today," he added.
Mr Yusuf alluded to the fact that it is likely Labour will lose their seat for the first time in more than a century describing it as a "catastrophe" for the party.
“They’ve never lost an election here... We know they will lose," he said.
Some people have said this is the most important by-election in history and Mr Yusuf believes we are witnessing a "total and historic realignment" of British politics in real time.
He said that the Caerphilly by-election would be a "big surprise".
"Turnout is up, turnout is going to be a big surprise."
He acknowledged there may have been some "tactical voting" in the election which could impact the result for Reform.
Mr Yusuf added: "But I’ll say this too, there’s a lot of people who did vote last night, who generally don’t vote, who are coming out to vote, which is really great."
Mark Palmer
Assistant editor, BBC Wales
Wales' deputy first minister Huw Irranca-Davies says it's been "a tough" by-election campaign for Labour with the party "playing the underdog here".
He tells the BBC Wales by-election special programme that on the doorstep, voters have been saying to him: "Look I’m Labour, but I’m lending my vote to Plaid Cymru because I don’t want Reform in."
Irranca-Davies says the party can get a "a compelling story" together for next May’s Senedd’s elections which could see the "polls shifting" in Labour’s favour.
"If we were to come, where the people are suggesting, in third place then that would be a real blow to us," he added.
He rejected that the party would go into a tail spin, but said they would rapidly reflect and refocus.
Oliver Slow
BBC Wales
Whatever the results are of tonight’s vote, the next few months will see huge changes in Welsh politics.
In May, voters will head to the polls for the next Senedd election, which promises to be the most consequential in Cardiff Bay since the National Assembly of Wales, as it was then called, was established in 1999.
This is largely because of major reforms to the way the Senedd is elected.
Not only will the number of Member of the Senedd (MS) increase from 60 to 96, but the way votes are counted will also change.
The current first-past-the-post system, where the candidate with the most votes wins the seat, will be replaced by a proportional system known as D'Hondt.
The method uses a mathematical formula to distribute seats based on the proportion of votes won by each party.
A brand new set of constituencies will also be created, with the number of MSs increasing from 60 to 96.
Forty of Wales’ MS are currently elected to represent individual constituencies, with a further 20 as regional MSs - four elected from each of the five electoral regions across Wales.
But under the new system, 16 larger constituencies will be created, with six MSs in each - bringing the number to 96.
David Deans
BBC Wales political reporter
It's still early in the count - but I've spoken to a couple of Plaid Cymru activists who are starting to feel positive about how the counting is going already.
The way the count works means you can see votes come in for different areas. Tellers from different parties sample the votes as they come in.
In one place where Plaid apparently did quite badly before, the party seems to be doing better now.
I've got to stress again - it's still really early.
Someone else I spoke to said they had been to too many of these things to get excited just yet.
Gareth Lewis
BBC Wales political editor
Image source, Mark LewisCounting is underway at the Caerphilly Leisure Centre
The postal votes in their big yellow tubs are being quickly emptied and counted.
There are about 20 black ballot boxes sealed with red cable ties in the middle of the sports hall.
There was a hush as counting got under way but the general hub-bub has resumed.
What really stands out at this count is the amount of London-based media.
They sense a story that could make the whole of the UK sit up and take notice.
Oliver Slow
BBC Wales
Throughout today, voters in Caerphilly have been voting to elect a politician to the Senedd in Cardiff Bay - but what is the Senedd, and what does it do?
The National Assembly for Wales, as it was then called, opened in 1999, moving certain responsibilities from Westminster to Cardiff Bay - although the assembly’s powers were initially fairly limited.
Over the years, however, further responsibilities have made their way down the M4, including full lawmaking powers in areas including health, education, transport, rural affairs and the environment.
The ability to raise and vary certain taxes has also now been transferred to what is now called the Senedd, or Welsh Parliament.
Other areas such as defence, benefits, foreign affairs and major rail infrastructure remain in Westminster.
Chris Mason
Political editor
Image source, Matthew HorwoodThe result is expected to be confirmed at about 02:00 BST
There is something about this moment that always quickens my pulse.
Polls close in an election, the counting is imminent.
Thousands of individual acts of decision accumulate and determine who will assume an elected office.
The thing with by-elections is they can have an outsized impact on the political weather.
Voters in a sliver of south Wales are picking a new member of the Welsh Parliament, the Senedd.
But in so doing they will also be sending a message to political leaders close and further afield.
A few thoughts for the hours ahead as we await the result:
How bad do things turn out for Labour? They have an up to now unblemished track record of victory in Caerphilly - in elections to the Welsh Parliament and to Westminster.
Many - including many within Labour - expect that run of wins to come to a crashing end tonight.
And then the big question beyond that: Who wins? Reform UK or Plaid Cymru?
This result will shape the mood of Wales’ political parties ahead of the Senedd elections across the country next May.
And they will shape the mood miles away in Westminster - as the Prime Minister Keir Starmer, the Reform leader Nigel Farage, Plaid Cymru, the Conservatives and others work out what the result tells them about how things are looking.
The power - right now at least - is in Caerphilly’s hands. And that is what democracy is all about.
David Deans
BBC Wales political reporter
Our team have been chatting to some well-placed Plaid and Reform sources who are both saying they think it could be close.
One even speculated about a recount.
Like we said earlier, they have just started counting so some of this might be based on gut instinct, and observations over the past six weeks.
The assumption among them seems that Labour would come third, in a by-election triggered by the death of party's Hefin David in the summer.
For their part, a Welsh Labour spokesperson has said: "Welsh Labour would like to thank everyone who came out to support our campaign and vote for Richard Tunnicliffe today.
"Hefin David loved this constituency and its people. He's been at the heart of our campaign throughout.
"This has been a tough campaign for the party for a variety of reasons, but we're proud of what our staff and volunteers have achieved over the past six weeks and are committed to continuing to fight for Caerphilly and the valleys."
Oliver Slow
BBC Wales
Image source, Getty ImagesHefin David was elected to Cardiff Bay in 2016
This by-election was called following the sudden death of the area’s MS, Labour’s Hefin David, in August.
Elected to Cardiff Bay in 2016, David was one of the more prominent Labour backbench members, making regular, lively contributions to Senedd debates.
He was also one of five Senedd commissioners, responsible for the day-to-day running of the institution.
Before becoming a full-time politician, David lectured at Cardiff Metropolitan University, teaching human resource management to undergraduates and postgraduates. He was also a Caerphilly councillor for a decade.
After his death, First Minister Eluned Morgan described him as a “much-loved member of the Labour family”.
“He was an outstanding politician, warm and enthusiastic and a great communicator,” she said.
“He will be greatly missed."
David Deans
BBC Wales political reporter
It's a pretty small leisure centre hall where they're counting the ballot papers tonight.
Richard Edmunds, the returning officer and the man responsible for ensuring tonight goes smoothly, has just announced the start of the first phase of counting.
The first big announcement we should hear is the turnout figure. They're just counting the number of votes in the first phase.
Eight politicians are competing in perhaps the most interesting by-election in the 26-years of Welsh devolution.The candidates, which you can read more about here, are:
Gareth Lewis
BBC Wales political editor
Image source, Getty ImagesIn the last Senedd election Labour won with a majority of 5,000 over Plaid Cymru
Multiple Labour sources have told BBC Wales that the party will lose the Caerphilly by-election.
It would bring an end to more than a century of Labour victories in the constituency in elections to Westminster and the Senedd.
Recent polling suggests that Plaid Cymru and Reform are fighting to challenge Labour’s dominance in Wales.
Sources at both parties say that the contest between them in Caerphilly is close.
Party leaders Rhun ap Iorwerth and Nigel Farage both campaigned there on Thursday.
The by-election comes just six months before the 2026 Senedd election and will be seen as one indication of what might potentially happen next May.
One Labour source described the expected defeat in Caerphilly as a “defeat for Starmerism not for first minister Eluned Morgan".
The first minister has been relatively popular on the doorsteps but the source warned that she needed to "assert herself" and differentiate Welsh Labour as much as possible from the UK party in the run-up to next May’s poll.
Oscar Edwards
BBC Wales
The count will now begin for control of the Caerphilly constituency
In the last few minutes the polls have closed in the Caerphilly by-election.
Counting will now begin and the results are expected at about 02:00 BST on Friday.
It will be seen as the final test of public opinion before the whole country goes to the polls next year.
National polling suggests Plaid Cymru and Reform are vying with Labour for the lead at next year's Senedd election.
Oliver Slow
BBC Wales
Image source, Getty ImagesLabour has dominated politics at the Senedd since devolution in 1999
Before Hefin David’s death, Labour held 30 of the Senedd’s 60 seats.
That meant it was already relying on the help of at least one other opposition member when it needed to pass legislation, or its spending plans.
Liberal Democrat Jane Dodds helped Labour out at the budget in return for a deal, including a ban on greyhound racing.
If Labour wins this evening the situation remains the same - if it loses it will need two opposition members to win Senedd votes.
Caerphilly is also part of an important symbol for Labour - it’s in their valleys backyard.
Losing would be a significant blow to the party, which has dominated politics in this part of south Wales since the early 20th Century.