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 said they will “test” what the law – expected to get final approval on Monday – requires of them.
Boris Johnson has been warned he could face legal action for flouting it.
The government has described the law – which would force the PM to seek a Brexit delay if MPs have not approved a new deal, or no deal, by 19 October – as “lousy”.
BBC political editor Laura Kuenssberg said although No 10 insisted it was not looking to break the law, efforts were under way to examine ways of getting around it.
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Two applications have been made to hold emergency debates in Parliament later.
Labour leader Jeremy Corbyn has requested a debate around the rule of law.
The second application is being made by Dominic Grieve – who lost the Conservative whip last week for voting to block a no-deal Brexit – but it is not clear what the subject will be.
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.