That would automatically trigger the Benn Act, which says the prime minister must ask the European Union for an extension if he hasn't finalized a deal with both the European Union and U.K. Parliament by October 19.
Johnson's Northern Irish allies will consider backing an amendment which would delay any immediate decision on the British prime minister's Brexit deal when it comes before parliament. Various scenarios may be put in motion by the vote.
Letwin says he backs Johnson's agreement but is concerned that lawmakers might fail to push through the required Brexit implementation mechanism by the October 31 deadline.
Parliament has also passed a law compelling the government to ask the European Union for a three-month delay to Brexit if a deal is not approved on Saturday. Johnson is expected to deliver a speech to parliament from 0830 GMT on Saturday, kicking off a day of debate that could last well into the evening.
"The momentum is with the prime minister, and momentum at a time like this is important", he said. The party has accused Johnson of selling out Northern Ireland, with deputy party leader Sammy Wilson calling it "toxic".
Once Parliament begins its sitting, Mr Johnson will make a statement to lawmakers, after which there will be a debate and then votes on amendments and finally - if all goes according to the government's plan - his deal. But around 20 of them, mainly representing pro-Brexit parts of the country, have previously indicated a desire to back a deal to honor the referendum result.
Northern Ireland's Democratic Unionist Party, which props up Johnson's minority government, will vote against it because of its arrangements for the British province.
That would be a major setback that would leave Brexit, yet again, up in the air as hundreds of thousands of protesters rally outside the 800-year-old parliament in central London to demand another referendum.
Johnson is seeking a clear-cut vote that either approves or rejects the divorce treaty.
Labour Party: Labour has 244 members of parliament.
"Voting for a deal today won't end Brexit".
Leader Jeremy Corbyn said he could not support the deal, and he will order his lawmakers to vote against it.
Labour rebels: Labour rebels are crucial to Johnson's hopes of getting a deal approved.
International Monetary Fund head Kristalina Georgieva urged caution while welcoming signs that Britain may reach a deal to exit the European Union.
Norman Lamb: while the rest of the Liberal Democrats are firmly against the deal, Lamb - who will step down at the next election - has always been much more Brexit-minded than his colleagues. They are made up of 35 Scottish National Party members, 19 Liberal Democrats and the rest from smaller parties or independents.
The 40-year-old premier's personal rating has shot up at any potential sign of success during the Brexit talks and the poll suggested he was by far the country's most popular leader. Parliament rejected her deal three times, by margins of between 58 and 230 votes earlier this year.
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