Defeat would trigger a law requiring him to ask European Union leaders to delay Brexit for what would be the third time. Though he is obliged by law to seek a Brexit delay if his deal falls, Johnson said the United Kingdom would still leave on October 31.
Last month Parliament passed a law compelling the government to do that if no deal is approved by Saturday.
Polish Prime Minister Mateusz Morawiecki said his country saw the vote as a delay, rather than a rejection of the Brexit deal.
The British government looks set to try again next week to get Parliament to back its divorce deal with the EU. She resigned in frustration after lawmakers repeatedly threw out her Brexit agreement. "This is them getting this kind of [Margaret] Thatcher-plus, deregulated economy that really will undo an bad lot of the things that Labour people believe in, and that's why I think it's disappointing if any Labour MP votes for this", Mr Campbell said.
The main opposition leader Jeremy Corbyn responded that "the prime minister must now comply with the law" and request an extension to Brexit until January.
Anti-Brexit demonstrators carry placards and European Union flags in London, Saturday, Oct. 19, 2019. Some demonstrators with bells strapped to their legs and wielding sticks performed a traditional morris dance and chanted: "Morris, not Boris!" to cheers from onlookers. That vote, which most analysts said would be tight, will now not take place until next week.
Elsewhere, the mood was less ebullient.
Parliamentarians in the United Kingdom voted to delay Brexit once again on Saturday after a landmark vote backing a motion called the Letwin Amendment.
Many carried signs calling for a halt to the Brexit process that started with a 2016 referendum in which British voters narrowly backed taking Britain out of the 28-nation bloc.
If attempts to avert a no-deal Brexit fail, the consequences for Britain are likely to be severe.
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Brussels urged Britain to explain its plan as soon as possible, while French President Emmanuel Macron's office said a new delay to Brexit was "in nobody's interest". "I don't believe new delays should be granted".
The deal covers Britain's financial settlement, protects the rights of European Union citizens and sets out a post-Brexit transition period potentially until 2022 to allow both sides to agree new trade terms.
Johnson has promised a hard Brexit if Parliament doesn't pass his plan, but economists say a no-deal breakup would plunge the UK's economy into recession.
He also warns it would hurt British manufacturing.
Johnson faces a close vote on his plan with opposition parties determined to block it.
Johnson called any delay to Britain's Brexit departure date "pointless, expensive and deeply corrosive of public trust".
"A vote for Letwin means MPs voting to render the entire day, that they demanded, meaningless". However the law is clear that he must send the letter Sunday. Those behind the amendment say it will remove the risk that the United Kingdom could stumble out of the bloc without a deal on October 31 because the law is not in place.
The vote is due later Saturday.
In a day of Brexit high drama, lawmakers will convene for the first Saturday sitting since the 1982 Argentine invasion of the Falklands, while hundreds of thousands of people march to parliament demanding another referendum.
In a divided parliament where he has no majority and opponents are plotting maximum political damage ahead of an imminent election, Mr Johnson must now win the support of 320 lawmakers to pass his deal through a booby-trapped legislature.
The Conservative Party confirmed the PM's stance in a tweet.