It has emerged that Nick Clegg met Gordon Brown in the Foreign Office on Sunday afternoon.
Labour and Lib Dem sources said that the meeting followed a telephone call between the two men last night and was intended to update each other on the situation surrounding negotiations to form a coalition government. Both sides described the discussion as "amicable".
Speculation had been mounting that all three party leaders were heading to a secret three-way meeting as Mr Clegg, David Cameron and Mr Brown were all seen on the move at once.
Negotiation teams from the Liberal Democrats and the Conservatives met for around six hours of talks on Sunday at the Cabinet Office, yards from Downing Street and the two sides have agreed to resume talks again on Monday.
The Prime Minister flew back to London from his constituency in Scotland on Sunday morning, amid warnings from his own Labour MPs that his position was becoming increasingly untenable.
In a defiant email to Labour Party workers thanking them for their efforts in the campaign, Mr Brown said he was determined to fight on to secure his policies for economic recovery.
"My resolve has not, and will not, change. I pledged to do everything in my power to fight for the people of this country - to secure the recovery, to protect their livelihoods and to continue to fight for a future fair for all," he said.
However Graham Stringer - long-time critic of Mr Brown - became the third Labour MP since the election to publicly call for Mr Brown to stand down, warning that he was losing support in the party.
"I've probably spoken to about 15 Labour MPs since the election - some of them who have been very supportive of Gordon over the last three years, some of whom have been closer to my position - and not one of them thinks he should stay on," he said.
John Mann, who yesterday became the first Labour MP to call on Mr Brown to go, warned that he was now an obstacle to any possible agreement with the Liberal Democrats.
"In the real world, Nick Clegg would be crucified if he propped up Gordon Brown. Gordon Brown's unpopularity was a key factor in this election. That's the reality," he said.
Other Labour MPs said that the party should accept that it had lost the election and give up its attempts to hang on to power.