Labour deputy leader Tom Watson is putting pressure on Jeremy Corbyn to work with the Liberal Democrats to stop a no-deal Brexit under Boris Johnson.
It comes as Lib Dem leader Jo Swinson said she would work with Mr Watson despite consistent criticism of the Labour leader over his Brexit stance.
Senior figures within Labour have ruled out working in a cross-party alliance to prevent no-deal as the Prime Minister prepares to leave the EU by October 31.
But Mr Watson took a different line as he spoke alongside Ms Swinson at an event by For Our Future's Sake and Our Future Our Choice, two youth movements campaigning for a second referendum.
"We cannot tolerate an undemocratic and disastrous no-deal being forced on the country by Boris Johnson," Mr Watson said.
"To stop him, everyone who cares about democracy, our country and our future must work together because there are enough of us - from all parties in Parliament - to stop him."
He added: "Whether you're Liberal Democrats, social democrats or democratic socialists, we are all democrats. And democrats have got to realise in this crisis that we're stronger together if we work together."
Shadow chancellor John McDonnell and shadow business secretary Rebecca Long-Bailey have both said Labour will not work in any pact with other parties.
Instead they say Labour can win a majority at a general election, which may be looming.
Ms Swinson has previously criticised Mr Corbyn as "indecisive" over Brexit, following calls for the Labour leader to firm up his party's stance towards a second referendum.
But at the event she said she would work with other parties to prevent a "disastrous" deal-less departure.
"Boris Johnson and his Cabinet are now actively pursuing that dangerous path, and I am committed to working with others across party lines in Parliament to stop no-deal Brexit," she said.
"This is no time for tribalism.
"That is why I am pleased to be here today with Tom to show that there is a desire across the political spectrum to stop Boris Johnson's reckless no-deal Brexit plan and instead deliver a brighter future inside the EU."
Mr Corbyn has said he will call for a vote of no confidence in the Government this autumn in an attempt to prevent no-deal.
