Why are the Liberal Democrats offering to help Boris Johnson secure a pre-Christmas election?

Jo Swinson has been accused of playing ‘old-style, selfish, tribal party politics’ but her party insists its priority is still trying to stop Brexit, writes Benjamin Kentish

Related video: Culture secretary Nicky Morgan dismissed the Lib Dem and SNP move as a ‘stunt’, but the government has suggested it could support it (PA)

It was on, then it was off, then it was on again. Now, Boris Johnson‘s bid to force an early general election looks set to succeed, thanks to the unlikeliest of allies.

The prime minister was handed a surprise boost at the weekend when the Liberal Democrats and SNP jointly announced that they were willing to support his calls for a pre-Christmas poll, despite the fact the two parties said the aim was to remove the Conservatives from power.

The two parties proposed a plan that would bypass Labour’s opposition to a December election, just as Jeremy Corbyn’s refusal to play ball had looked set to scupper Johnson’s ambitions.