Jeremy Corbyn and other opposition leaders have agreed to try to stop a no-deal Brexit through legislation after efforts to install a caretaker prime minister floundered.
Shadow Brexit secretary Sir Keir Starmer has said it was time to “put aside fantasy politics” and find a strategy to enact “straight away” when parliament returns.
It comes as Boris Johnson prepares to create half a dozen new pro-Brexit peers to redress perceived Remain bias in the House of Lords.
The PM is sending his “sherpa” David Frost to Brussels this week to try to negotiate an alternative to the backstop with the EU.
Mr Johnson also told Jean-Claude Juncker, the European Commission president, that Britain will leave without a deal unless the backstop is “abolished”.

Jeremy Corbyn’s letter urging MPs to unite to stop no deal Brexit – in full
Labour leader calls for ‘practical way' to stop UK leaving EU without a deal
Sketch: Nigel Farage is howling about democracy, like every other miniature demagogue that has gone before him
Nigel Farage is back and he's angrier than ever, angry that his lies can never come true
Boris Johnson to appoint string of Brexit supporters to House of Lords
Nigel Farage is not expected to be among those honoured for their part in the Leave campaign
Jeremy Corbyn: I will do everything I can to stop this no-deal bankers’ Brexit
Exclusive: The battle to stop no deal is the fight of the many against the few who are hijacking the referendum result to hand even more power to the wealthiest
Boris Johnson suspending parliament to force no-deal Brexit would be 'greatest abuse of power in living memory'
Shadow attorney general says attempt to prorogue parliament to stop MPs blocking no-deal would likely be thrown out by courts