Key allies of party leaders Nick Clegg and Ed Miliband have met to discuss what the two parties have in common, BBC Newsnight has learned.
The Labour peer Lord Adonis, and Lord Wood, one of Ed Miliband's closest advisers, met Jonny Oates, Clegg's chief of staff and Neil Sherlock, a prominent Liberal Democrat and donor.
The men met for dinner in April.
Members of the group deny the meeting constituted any kind of formal talks about a potential coalition government.
But others believe the meeting is significant as it suggests the two parties are considering how they might prepare for potential coalition in the event of a hung parliament at the 2015 general election.
Lord Adonis was part of the Labour team who tried and failed to form a coalition with the Liberal Democrats after the last general election.
The two parties have until now, tried to keep any contact over a potential future coalition deal secret.
Members of the group insist the dinner, which took place at the Joe Allen restaurant in central London, was an "informal catch up between friends who have known each other for years".
But one member of the group admits at least that the parties' common ground on the European Union was discussed.
The group was brought together for a discussion because of "mutual concern over Europe", BBC Newsnight has been told. And the programme understands a wider range of issues was discussed.
Beyond this particular dinner, a senior Labour source now admits that there are a "whole range of informal contacts all the time" between the two parties, and that many are made "through Vince" - the Business Secretary Vince Cable.
Last week, Mr Cable was under fire for the behaviour of his friend, Lord Oakeshott, who made a bungled coup attempt to oust Nick Clegg. Mr Cable is considered a popular figure on the left of the Lib Dems.
Newsnight also understands there have been recent contacts between Nick Clegg and Ed Miliband, following their co-operation on the Leveson Report.
The Liberal Democrats have recently established a coalition "preparation group", led by Danny Alexander to get ready for potential negotiations that may take place after a general election, irrespective of which party wins most seats.
The Liberal Democrats would again, it's understood, begin talks first with the party with the largest number of seats, in the event of no party having an overall majority.
One Liberal Democrat source told Newsnight that despite four years of coalition, it would be "easier to build a platform" with Labour than with the Conservatives after the next election.
Relationships between the two parties have thawed since the first couple of years of the coalition when Labour continually described the coalition as the "ConDems".
But one minister suggested doing a deal with Labour would be "as close to a nightmare as you can imagine".