Coalition harmony has faced fresh setbacks this weekend amid speculation over the extent of Liberal Democrat links with the Labour party.
Business secretary Vince Cable has been forced to admit that 'informal' talks have taken place at 'different levels' between left-leaning Lib Dems and Ed Miliband's senior colleagues, the Sunday Telegraph reported.
Conversations over issues where the parties have a shared interest, like Lords reform, are suspected by Tory backbenchers of being used as a cover for discussions about longer-term prospects.
A Labour source told the newspaper: "These talks are designed to find common ground so that, if the next election result is inconclusive once again, we could enter a progressive coalition with them."
The suggestion of links between the parties prompted Labour's former transport secretary Andrew Adonis to acknowledge that he has kept up strong links with Lib Dems.
"The problem they have got is the leadership of their own party is stuck in with the Conservatives and it's very difficult to extricate yourself when you are in a coalition like this," he told the Today programme.
"But I think there are a lot of Lib Dems that would rather not be where they are at the moment."
Other left-wing Lib Dems said to be involved in the informal talks with Mr Miliband include the party's president, Tim Farron, former leader Sir Menzies Campbell and deputy leader Simon Hughes.
Lord Adonis had called for Lib-Lab talks to start immediately in an interview with politics.co.uk in September 2011.
"Having a dialogue with the Liberal Democrats is important and it can take many different forms," he said.
"It can take the form of a quasi-public dialogue as it did in the case of phone-hacking, but there's a lot of private dialogue that needs to take place too.
"I hope that will strengthen the progressive side of politics and in due course maybe promote some new forms of Lib-Lab partnership."