Downing Street has indicated it will back a plan by the Liberal Democrats to change the law in order to hold a December election if Labour refuses to back Boris Johnson’s motion for an early poll.

A No 10 source attempted to put pressure on Labour to back its motion under the Fixed-term Parliaments Act for an election on 12 December, saying the government would turn to the Lib Dem plan as an alternative.

Johnson needs Labour support to get a two-thirds majority for an election under current legislation, and Jeremy Corbyn is refusing to back one while the possibility of a no-deal Brexit is still alive.

Over the weekend, the Lib Dems offered to help Johnson change the law to allow an election on 9 December, which would need only a simple majority in parliament.

A No 10 source said: “Tonight is Labour’s last chance to have an election with Brexit delivered – they can vote for the 12th and get Brexit done before. If not, we will introduce a bill almost identical to the Lib Dem/SNP one tomorrow.

“This parliament has repeatedly failed to deliver on its promise to respect the referendum. Millions of families and businesses can’t plan because of constant delays. We need a new parliament by Christmas so we can get Brexit done in January and the country can move on.”

It is not clear how the government’s proposal for a bill would differ and it is possible that No 10 could be using the idea to put pressure on Labour before tabling legislation that the Lib Dems could not back.

However, Lib Dem sources indicated they would be willing to support any legislation that was fundamentally similar to their own and confirmed they would not pursue amendments in order to ensure the bill had the best chance possible of getting through.

It is understood that the bill would need to be tabled on Tuesday and passed by Thursday in order for a 9 December election to take place.

Corbyn suggested on Monday that his party could abstain on the government motion on Monday but back the Lib Dem plan.

“We will consider carefully any legislation on an early election,” he said, adding that a date needed to be locked down in law to prevent Johnson from trying to move it for his own advantage.

Corbyn also suggested he would want an election to be held earlier than 12 December and before students break up for the winter holiday, saying any plan would need to “protect the voting rights of all of our citizens”.

Earlier, John McDonnell, the shadow chancellor, framed the situation as “the Lib Dem and Tory pact of 2010 being re-established”. He said: “They are back together, selling out the People’s Vote campaign and the cross-party campaign to prevent a no deal. The Lib Dems will stop at nothing to get their ministerial cars back.”

Jo Swinson, the Lib Dem leader, hit back at McDonnell, tweeting: “In the last 10 days I’ve addressed a #PeoplesVote March and tabled an amendment to the Queen’s speech that would have given us a people’s vote. Your leader did neither. Don’t lecture me about fighting for a people’s vote.”