General election: Labour fury as 'sell out' Lib Dems offer to help Boris Johnson secure pre-Christmas poll

John McDonnell among senior figures attacking Jo Swinson after she joins forces with SNP to back government calls for December election

Labour has launched a furious attack on the Liberal Democrats amid a row over Boris Johnson's attempts to force a pre-Christmas general election.

Senior figures in Jeremy Corbyn's party condemned Lib Dem leader Jo Swinson after she joined with the SNP in ditching an opposition party agreement to oppose a snap election and offered to help the prime minister secure a December poll.

Labour is expected to vote against Mr Johnson's attempt to force an election when MPs vote on the matter on Monday and says it will not support a poll until a no-deal Brexit has definitely been avoided.

Mr Johnson's motion is likely to fail because, under the Fixed-term Parliaments Act (FTPA), two-thirds of MPs would need to support it for a poll to be triggered.

The prime minister is then expected to try to introduce a one-line bill saying the FTPA should be amended to allow an election in early December.

It followed a joint move by the Liberal Democrats and SNP proposing such a plan for an election on 9 December and suggesting they would support the bill if the government introduced it.

Ms Swinson said she now accepted that the current Parliament would never support plans for another Brexit referendum and that an election was needed to break the current deadlock.

That prompted an angry response from Labour. John McDonnell, the shadow chancellor, accused Ms Swinson of "selling out" the campaign for another referendum.

He tweeted: "Looks like the Lib Dem and Tory pact of 2010 is being re-established. 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."

And Ian Lavery, the chair of the Labour Party, said: "Politics is a strange business. It really is. So the revoke Article 50 Lib Dems want to get into bed with the no deal Brexit Conservatives and forget their chums in the PV campaign. Moral of the story: never trust a Tory and for heavens sake never trust the Lib Dems."

But Ms Swinson hit back, replying to Mr McDonnell saying: "In the last 10 days I've addressed a People's Vote 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.

"We will keep fighting for a People's Vote, but unless Labour wholeheartedly back it then a general election is the only way we can use this extension to #StopBrexit."

A senior Liberal Democrat source added: "It is absurd to suggest that the Liberal Democrats have given up on the principle of a People's Vote."

They suggested that Mr Corbyn was "not serious" about a second referendum and added: "Let's face the facts - at the moment there are not the numbers for a People's Vote in this parliament."

The Liberal Democrats believe that the EU is unlikely to grant a further Brexit extension beyond the 31 January and that there is therefore a major risk of the EU crashing out of the bloc on that date.

They say the risk of Mr Johnson securing parliamentary approval for his Brexit deal is now greater than the risk of the Conservatives winning a majority at an early election.

Lib Dem sources suggested the party was likely to support a government bill for an early election as long as three conditions were met: the date of the poll was enshrined in law, attempts to enshrine Mr Johnson's Brexit deal in law were halted, and the risk of a no-deal Brexit before 31 January was removed.