Jeremy Corbyn risks jeopardising a vote of no confidence in the government by insisting he becomes caretaker PM, Lib Dem leader Jo Swinson has said.
If he wins a no-confidence vote, the Labour leader plans to form an emergency government and then delay Brexit to avoid a no-deal scenario.
But in a new letter, Ms Swinson said Mr Corbyn's insistence on being interim leader meant there was a danger not enough MPs would support the vote.
Labour did not respond to the letter.
Instead, the party referred to comments made by its shadow international trade secretary, Barry Gardiner, who on Sunday described Ms Swinson as "extremely petulant" for dismissing Mr Corbyn's initial proposal to lead a temporary government.
Prime Minister Boris Johnson has repeatedly insisted that the UK will leave the EU on 31 October, with or without a deal.
On Sunday, he told the BBC the chances of securing a new Brexit deal were "touch and go", having previously said the odds of no deal were "a million to one".
Ahead of cross-party talks on how to avoid no deal - due to take place on Tuesday - Ms Swinson said the discussions should examine how to seize control of Commons business, oust Mr Johnson and install an emergency "government of national unity".
In her letter to Mr Corbyn, Ms Swinson added: "Insisting you lead that emergency government will therefore jeopardise the chances of a no confidence vote gaining enough support to pass in the first place.
"As you have said that you would do anything to avoid no deal, I hope you are open to a discussion about how conceding this point may open the door to a no-confidence vote succeeding. Its success must be the priority."
Earlier this month, Mr Corbyn outlined his plans to avert a no-deal Brexit - which involve him becoming a caretaker prime minister - but was met with resistance from some key potential allies.
Ms Swinson and Conservatives opposed to no deal were among those who rejected the idea of Mr Corbyn being interim leader, but Scottish First Minister Nicola Sturgeon criticised the Lib Dem leader's stance, adding that "nothing should be ruled out".
Mr Corbyn said he would call a no-confidence vote at the "earliest opportunity when we can be confident of success". That cannot happen before 3 September, when MPs return from summer recess.
In order for such a vote to succeed, Labour would require support from across the House of Commons, including the Lib Dems, the SNP and Conservative rebels.
Ms Swinson has suggested Tory MP Ken Clarke and former deputy Labour leader Harriet Harman as possible caretaker leaders, and both have expressed willingness to do the role, she said.
In her letter, Ms Swinson also called on Mr Corbyn to "clarify" his position on whether he was opposed to Brexit altogether.
The SNP, Liberal Democrats, Change UK, Plaid Cymru and the Green Party have all accepted the invitation to meet Mr Corbyn to discuss proposals for an alternative government to be formed when Parliament returns in September.
Speaking ahead of the meeting, Labour's Barry Gardiner told Sophy Ridge on Sky News on Sunday Labour was offering a "failsafe procedure to stop no deal" by holding a vote of no confidence followed by a temporary government to set up a general election.
Former Prime Minister Gordon Brown told BBC Radio 4's Today programme: "I think it is more likely Parliament will force Boris Johnson to delay a no deal and the question is whether in these circumstances he will think it is the right thing to call an election.
"My view is that an election at this point will not solve the problem we have - and the problem we have is making sure that we do not inflict harm on ourselves, by leaving the EU with no deal."