In order to trigger an early election, Boris Johnson will need the votes of at least 434 MPs.

But even if the government loses, there are still other routes to a possible pre-Christmas election.

You can read about these here.

House of Commons
Parliament

Jeremy Corbyn accuses ministers of misrepresenting the contents of the PM's "rejected and recycled" Brexit deal.

The government, he says, "simply cannot be trusted" to keep high standards on workers' rights and the environment.

Mr Corbyn says he does not trust the prime minister to call an election on 12 December if MPs back the motion tonight.

He says the UK has "not accepted" the EU's request for a three-month Brexit delay, for which he says "legal confirmation" has not yet been secured.

House of Commons
Parliament

Responding for Labour, the party's leader, Jeremy Corbyn, says Boris Johnson "cannot be trusted".

He accuses the prime minister of "abandoning" attempts to get his deal through Parliament.

And he says the PM has "failed to deliver" with his pledge to make Brexit happen by 31 October - for which Mr Johnson is seeking to blame Parliament.

House of Commons
Parliament

Mr Johnson tells MPs that under Jeremy Corbyn there will be the "toxic, tedious torture of two more referendums".

He adds: "It is time for the leader of the opposition to move his rusty Trabant from the junction where it is blocking progress.

"It is time to faces the bosses - the people."

House of Commons
Parliament

Boris Johnson says Jeremy Corbyn has "run out of excuses" not to vote for a general election.

"He can run but he cannot hide forever," he adds.

"Across Parliament, his supposed allies are deserting him," he says, adding that the SNP and the Lib Dems are now in favour of an election.

The PM says: "There is one party tonight that is against a general election, there is one party who does not trust the people.

"He is snookered and this charade has gone on long enough."

BBC News Channel

Kirsty Blackman, the SNP's deputy Westminster leader, says the difference between 9 December and 12 December is "crucial".

She tells the BBC News Channel: "It is an absolutely crucial three days because actually what it means is the Brexit bill cannot go through Parliament. Basically it means we are safe and cannot possibly be dragged out of the EU against our will."

She adds: "We want to ensure this general election happens in time that we can't have that Brexit inflicted upon us... we are trying everything that we can to stop it."

Boris Johnson has written to European Council President Donald Tusk to formally accept the EU's offer to delay Brexit to 31 January 2020.

In his letter, he says he has "no discretion" under UK law to otherwise - saying an extension is "unwanted" and "damaging to our democracy".

He says he would have "much preferred" if Parliament had ratified his Brexit deal, but he fears MPs will "never do so" as long as "further delay" remains an option.

He asks the EU to "make clear" that another extension beyond 31 January will not be granted if MPs fail to back his call for a December election.

House of Commons
Parliament

Boris Johnson tells MPs his proposed date of 12 December for an election provides "a hard stop that everybody can believe in".

He says that without that "hard stop... we are all like Charlie Brown, endlessly running up to kick the ball, only for Parliament to whisk that ball away."

House of Commons
Parliament

The PM says Parliament almost did "the sensible thing" by voting for the Brexit bill at second reading, but then "threw it out" by voting against the the timetable he had proposed.

"They made it inevitable that the people of this country would be retained in the EU against their will for at least three months at a cost of another £1bn," he says.

House of Commons
Parliament

The PM begins by saying that "nobody in this House relishes the idea of a general election".

He adds: "Nobody wants to put the public to this inconvenience, particularly in this season.

"But there is a widespread view that this Parliament has run its course."

House of Commons
Parliament

The PM gets to his feet to open the debate on holding a general election.

The debate on his election motion is due to start any moment...

Labour MP Diane Abbott has tweeted to clarify earlier reports about comments she made during a shadow cabinet meeting.

Professor Sir John Curtice
Polling expert

It is not a risk free strategy but going for an election has become an easier decision for the government to make in the last two weeks.

The truth is the opinion polls have been suggesting that the Conservative lead over Labour has been edging up.

Even on a relatively conservative calculation, it now stands at 10 points, rather than seven points at the back end of September.

And actually all of the most recent polls suggest the lead is in double figures.

The reason why that matters is once the Conservatives have got a double digit lead, it is almost inevitable they are going to get an overall majority, whereas at seven points it was a 50/50 shot.

That said, there are downsides.

One is that it assumes that the squeeze on the Brexit Party vote - the foundation upon which Boris Johnson's lead rests - continues, goes a bit further and certainly isn't reversed by the arguments that will come from Nigel Farage.

He will say that we should have just left without a deal and this is the third time the Conservatives have failed to deliver Brexit.

We have not seen or heard a lot from Nigel Farage and the Brexit Party of late, but once the rules for equal time for broadcasting kick in, we will hear rather more.

Read the BBC's latest opinion poll tracker