Jeremy Corbyn has been warned he faces the prospect of defections by MPs as Labour struggles to come to terms with his leadership.
Liberal Democrat leader Tim Farron claimed he had received "unsolicited texts" from well-known figures "distressed" about the direction the party was taking.
The comments came after a millionaire donor offered to support MPs who want to break away from Labour.
Businessman and Hull City owner Assem Allam, who has donated £720,000 to Labour since 2010, said Mr Corbyn could not provide a "strong opposition" and warned the party could face three further election defeats.
The Labour leader was seen as having performed well against David Cameron at Prime Minister's Questions, and has appeared to soften his stance on a key policy positions such as support for EU membership in an attempt to unify the party.
Mr Corbyn also indicated he would not attempt to enforce his personal opposition to the Trident nuclear deterrent or welfare cap.
However, the leader has yet to finalise his frontbench team five days after he started making appointments, and some critics have been predicting that he may not manage to fill all the posts.
Speaking to the Evening Standard, Mr Farron - whose party was reduced to a rump of just eight MPs at the general election - said: "I've had various unsolicited texts, some of them over the weekend, where I felt like I was being an agony aunt rather than anything else.
"People who have been members of the party for as long as I've been a member of mine who feel that they don't recognise their party any more and feel deeply distressed."
Mr Farron said "some of them" had been well-known figures, and pressed on whether they included frontbenchers he replied: "I couldn't possibly comment. The bottom line is ... people in the Labour Party need to understand they can have conversations with me, which may or may not be conclusive, which will remain totally between me and them."
Former foreign secretary Jack Straw said Mr Corbyn had done well at PMQs yesterday.
But he warned that the leader and shadow chancellor John McDonnell faced "huge personal as well as political challenges" and there was an "onus" on them to move towards the centre of the party.
"We will just have to see how it works out," Mr Straw told BBC Radio 4's World At One. "Both John McDonnell and Jeremy Corbyn, given the fact that they both had always in the past been on the backbenches and not accepted any kind of collective responsibility, have huge personal as well as political challenges in front of them.
"I hope for the sake of the party and country that they will be able to meet them. Time will tell."
He added: "People have very different views in that shadow cabinet. I think the onus obviously is on Jeremy Corbyn and John McDonnell that they will have to move towards the centre of gravity of the parliamentary party which in turn represents the centre of gravity of Labour voters.
"Whatever people say about MPs, if you are an MP of any party you have a pretty shrewd idea of where your voters' interests lie."
Mr Straw played down the prospect of MPs defecting to other parties, suggesting the experience of the SDP in the 1980s made that "very unlikely".
However, he cautioned that the new leadership could have a "Lazarus effect" on the Liberal Democrats.
Mr Corbyn is expected said to be planning to take the fight to Mr Cameron during the Tory conference by breaking usual etiquette and making a speech to a TUC event just one mile away from the venue in Manchester.