When a vote is held in the House of Commons, the speaker asks MPs to call out whether they are for or against the bill. If there is not a clear winner, he calls a vote, known as a division.

MPs then go into one of the two rooms at the end of the Commons chamber - the division lobbies - and have to walk through to cast their vote.

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The result of the vote on Sir Oliver Letwin's amendment is expected at about 14:45 BST.

A man has been arrested for trespassing at the Palace of Westminster.

A spokeswoman for the House of Commons confirmed an "incident" had taken place involving a visitor to the Parliamentary estate.

"Security staff and the police attended and the situation has been resolved," she said.

MP for Oxford West and Abingdon tweets...

The shadow Brexit Secretary warned MPs against the prime minister's agreement

Sir Nicholas Soames was among the 21 Conservative MPs who were kicked out of the Conservative Party by Boris Johnson in September for rebelling and voting to prevent a no-deal Brexit.

He's back on board and supporting the prime minister in today's vote.

The now-independent MP, who is Winston Churchill's grandson, tweets: "I shall vote against the Letwin amendment and for the deal."

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The Guardian's Dawn Foster says "it looks as if the Labour rebellion is going to be smaller than the government had expected" for the Brexit deal vote.

She says: "The Conservatives were briefing that they could pick up 19.

"But Jeremy Corbyn and John McDonnell spent almost all of last night calling round Labour MPs who might be considering rebelling, trying to bring them back on side.

"Eleven have come out firmly now and said that they will. Twelve or 13 is probably the highest we are looking at.

"Overall, I think this could lose by a very small handful."

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The House of Commons is voting on independent MP Sir Oliver Letwin's amendment to the government's motion.

This would withhold parliamentary support for the government's deal unless and until legislation implementing the agreement in UK law is passed by MPs.

If it's passed, it could force the prime minister to seek a further delay to Brexit.

Getting bogged down with amendments and motions and just want to know what this means for the bigger Brexit picture?

Our senior political analyst Peter Barnes has put together a helpful explanation - with flowcharts showing possible scenarios.

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Chancellor of the Duchy of Lancaster Michael Gove - winding up the debate for the government - says many MPs will have "qualms and concerns" but the "time has come for us to decide" on Brexit. If the Commons doesn't support the PM's deal, voters will feel it has "chosen to duck its responsibilities".

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Katy Balls, deputy political editor of the Spectator, says a lack of support for the Brexit deal by DUP MPs means the government needs "significantly more" Labour MPs to vote in favour of the deal.

"For every DUP MP you don't have, you need a Labour MP," she says.

"There is a sense that the government is quite happy that the majority of the European Research Group have come out in support of the deal.

"But if they have any chance of this passing they have to get more Labour support and at the moment it seems we are in the 12 or 13 mark and it needs to go up a lot."

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Boris Johnson's deal will add costs and bureaucracy for UK businesses, Labour's Rebecca Long-Bailey tells MPs. It will also damage workers' rights, she adds.

If the Letwin amendment passes, it is "too late" for the government to pull the main vote on the deal, says BBC political editor Laura Kuenssberg.

But it can be "negatived", she says - ie if the government doesn't provide tellers or no-one shouts "aye" on the second call then the vote could just come to a halt.

She adds that Downing Street is suggesting the Letwin amendment increases the chances of no-deal happening.

The Letwin amendment would provisionally approve Boris Johnson's Brexit deal, but withhold official support and delay Brexit until the necessary legislation is passed.

Our correspondent adds:

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Shadow business secretary Rebecca Long-Bailey says the PM's deal doesn't protect UK communities. It would be a "disaster for this country" if MPs backed it, she adds.

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Anna Soubry, leader of the Independent Group for Change, says the Leave campaign "played upon people's prejudices" during the referendum campaign. The only way to heal divisions now is to hold another public vote, she tells MPs, arguing that "people are entitled to change their minds".

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Independent MP Antoinette Sandbach says she will support Sir Oliver Letwin's amendment, and that she has "nothing nice to say" about Boris Johnson's deal.

The SNP's Westminster leader Ian Blackford said Scotland was being "totally and utterly shafted" by the PM's Brexit deal.

The former prime minister backs the Brexit deal, and urges MPs to support it.

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Conservative MP Sir Bill Cash says he will back the PM's deal, calling this a "very, very historic moment" and arguing that the UK must leave the EU on 31 October.

BBC assistant political editor Norman Smith tweets...