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.
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.
One plan reportedly under discussion to get round the Brexit delay legislation is to ask a sympathetic EU member to veto an extension.
Another potential option would be to formally send the extension request mandated by the new law, but also send a second letter to the EU making it clear the UK government does not want one.
However, Lord Sumption, a former judge of the UK’s Supreme Court, said such a ploy would not be legal.
“To send the letter and then try and neutralise it seems to me to be plainly a breach of the act,” he told BBC Radio 4’s Today programme.
Mr Johnson argues he has been forced into seeking an election because the no-deal delay bill makes further negotiation with the EU pointless.
But the election motion, which requires the support of two-thirds of MPs, was defeated last week and is expected to fail again.
Downing Street has accused Labour of denying the public the right to have a say and is arguing that Jeremy Corbyn should take the opportunity to seek his own mandate from the public to delay Brexit.
But the Labour leader told reporters earlier: “I think it is extraordinary that we have a prime minister who has lost every vote he has put to Parliament in the few days it has been back that now goes around the country saying that he is now going to defy Parliament.
“Democracy requires that elected governments are responsible to Parliament itself and the prime minister seems not to be prepared to do that.”