The walk-out would prevent a highly symbolic defeat for the prime minister on the withdrawal agreement he sealed with the remaining 27 EU states on Thursday.
Mr Johnson has appealed to MPs to give their endorsement to the deal in a fourth “meaningful vote” in the Commons today, after his predecessor Theresa May went down three times to defeat.
But an amendment tabled by former Tory cabinet minister Sir Oliver Letwin would withhold the House’s approval until legislation to ratify the deal has completed its passage through parliament.
If passed, the Letwin amendment would force Mr Johnson to send a letter to Brussels asking the EU to extend Brexit beyond the current deadline of 31 October.
Downing Street declined to say whether the PM would sign the letter by the end of the day, as required by the so-called Benn Act, instead repeating its mantra that “we comply with the law but we are leaving on 31 October”.
The Letwin measure looks almost certain to pass, with the backing of Labour, the Democratic Unionist Party and other opposition parties as well as the bulk of the 21 Tories expelled over a previous rebellion.
Mr Johnson himself told the Commons that he “fears” it will prevent today’s government motion from fulfilling the legal requirement for a “meaningful vote” before Brexit, making the final division pointless.
But parliamentary rules deny the Government the option of pulling its motion, leaving Mr Johnson no choice but to order a Tory boycott or risk seeing them outvoted.
The vote is on a razor’s edge, with Johnson desperately trying to talk over the dozen or more Labour MPs he needs after the DUP rejected his deal. A walk-out would fuel speculation that the prime minister does not believe he has the numbers to get the agreement through parliament.
In a day of high drama in Westminster, the PM told MPs he hoped that the first Saturday sitting of the Commons since the Falklands War would see them show the “ability and resolve to reach beyond past disagreements by getting Brexit done and moving this country forwards, as we all yearn to do”.
Speaking as hundreds of thousands of second referendum supporters gathered for the giant Together for the Final Say march on the streets outside, Mr Johnson said that that Thursday’s agreement represented “the best possible solution” to Brexit and urged MPs to “come together as democrats to end this debilitating feud”.
He warned that there was little appetite among EU nations for further delay, which he said would be “pointless, expensive and deeply corrosive of public trust“.
“Let’s go for a deal that can heal this country and allow us all to express our legitimate desires for the deepest possible friendship and partnership with our neighbours,” he said.
But Labour leader Jeremy Corbyn said that Johnson had “re-negotiated the withdrawal agreement and made it even worse”.
Restating Labour’s comment to put any Brexit deal to a Final Say referendum, Mr Corbyn told MPs: “Voting for a deal today won’t end Brexit. It won’t deliver certainty and the people should have the final say.
“Labour is not prepared to sell-out the communities we represent. We are not prepared to sell out their future. And we will not back this sell-out deal.”
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