The Liberal Democrats and the Scottish National Party have joined forces to back a change in the law to try to trigger a general election in December.
The prime minister has already proposed holding an election on 12 December but needs two thirds of the 650 MPs in parliament to back his plan in a Commons vote on Monday.
It will be his third attempt at getting support for the UK to go to the polls.
Most opposition parties, including the Lib Dems and SNP, have opposed the idea but the two pro-Remain parties signalled on Saturday evening they would back a December election if it was separated from Brexit legislation.
In a move that circumvents Labour’s opposition, the Lib Dems and SNP are set to offer Mr Johnson an even easier route to a snap poll, requiring just a simple majority in the Commons.
If the European Union gives the UK a Brexit delay until 31 January, as requested in Mr Johnson’s letter to Brussels last week, the Lib Dems and SNP are prepared to give the PM the opportunity to have an election – but on new terms, not Mr Johnson’s.
Ian Blackford, the SNP’s Westminster leader, confirmed he had co-signed a letter with Lib Dem leader Jo Swinson to European Council president Donald Tusk in which they sought an extension until 31 January at the earliest so that the “risk of a devastating no-deal Brexit” could be removed.
He said: “If that meaningful extension is secured we will then work together to bring forward an election this year – but on parliament’s terms, not on the prime minister’s.
“This is the only way to unlock what has become a deadlocked parliament and for the UK and the EU to move forward.”
The two parties’ MPs have drawn up a bill that would allow Mr Johnson to secure a December election with a simple majority of MPs, by-passing the need for two-thirds support.
The draft law would grant an election on 9 December – three days before the PM’s proposed date and, crucially, when more students are still at university to cast their votes in Remain-supporting target swing seats.
The move indicates a split between opposition leaders on whether to go for a December campaign, with Labour putting off their decision on how to vote on Monday as they trail the Tories in opinion polls.
Jeremy Corbyn said he wanted to wait until EU ambassadors had finalised their decision, expected on Monday or Tuesday, over the length of an extension for Article 50.
Speaking on a campaign visit on Saturday, the Labour leader said: “We will be very happy to fight an election once all vestiges of a no-deal exit from the EU have been taken off the table.”
Britain is due to leave the EU on 31 October but parliament has passed a bill forcing it to secure a delay to prevent the country from leaving without a deal to smooth the process.
The prime minister used an article in the Sunday papers to accuse MPs of holding the country “hostage” by refusing a general election.
Outlining his fear that the Commons will “waste the next three months like it’s wasted the last three years”, Mr Johnson is reiterating his “reasonable offer” that, in exchange for more time to consider his Brexit deal, MPs must back a 12 December election.
A new Opinium poll published on Saturday saw the Conservatives increase their lead over Labour as they moved to 40% support.
Mr Corbyn’s party remained at 24%, with the Lib Dems on 15%.
Sophy Ridge will be talking to Lib Dem MP Chuka Umunna about the potential general election and all things Brexit from 8.30am this morning on Sky News.