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 said they would "test to the limit" what it required 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, which was 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.
Meanwhile, John Bercow has said he will stand down as Commons Speaker and MP at the next election, or on 31 October, whichever comes first, after 10 years in the role.
Two emergency debates are taking place in the Commons, ahead of a vote on a general election.
MPs have begun debating an application from Dominic Grieve - now sitting as an independent after losing the Tory whip - who is seeking to ensure the publication of government communications relating to the suspension of Parliament.
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.
Mr Grieve told MPs it was "entirely reasonable" to ask for the disclosure "so the House can understand the risks involved and this can be communicated more widely to the public".
Labour's shadow Brexit secretary, Sir Keir Starmer, said there was a "basic lack of trust... between this House and the executive".
"It's blindingly obvious why we're being closed down - to stop scrutiny and to prevent this House expressing a view on no deal," he said.
"It is not just us that is being shut out, it is the people we represent."
