The five-week suspension of Parliament will begin later, after MPs are expected to again reject government calls for a snap election.
Opposition MPs confirmed they would not back the push for a 15 October poll, insisting a law blocking a no-deal Brexit must be implemented first.
Ministers have called the law “lousy” and say they will “test to the limit” what it requires of them.
Boris Johnson has been warned he could face legal action for flouting it.
At present, UK law states that the country will leave the EU on 31 October, regardless of whether a withdrawal deal has been agreed with Brussels or not.
But the new legislation, due to be granted royal assent on Monday, changes that, and will force the PM to seek a delay to 31 January 2020 unless a deal – or a no-deal exit – is approved by MPs by 19 October.
BBC political editor Laura Kuenssberg said although No 10 insisted it was not looking to break the new law, efforts were under way to examine ways of getting around it.
Two applications have been made to hold emergency debates in Parliament later – the first, from Labour leader Jeremy Corbyn, concerns the rule of law.
The second is being made by Dominic Grieve, who lost the Conservative whip last week. He wants all internal communications between a group of No 10 aides, including Mr Johnson’s controversial adviser Dominic Cummings, about Parliament’s suspension to be made available to MPs.
He is also pressing for full disclosure of all documents relating to Operation Yellowhammer, the government’s no-deal contingency plan, shared with ministers since 23 July.
Meanwhile, Downing Street confirmed that the expected prorogation – or suspension – of Parliament until 14 October would begin at the end of Monday’s sitting.
It means MPs will not get another chance to vote for an early election until after then, meaning a poll would not be possible until late November at the earliest.