The five-week suspension of Parliament will begin later today, 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.
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.
The new legislation, due to get royal assent later, 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.
Ministers have called the new law “lousy” and say they will “test to the limit” what it requires of them, but PM Johnson has been warned he could face legal action for flouting it.
Although No 10 insisted it was not looking to break the new law, efforts were under way to examine ways of getting around it.