Lord Ashdown discussed forming a coalition with Labour in the past
Senior Labour MPs have discussed defecting to the Liberal Democrats if the party loses the election, Lord Ashdown has said.
In a Daily Telegraph interview, the former Lib Dem leader indicated some in Labour were concerned about a lurch to the left in the event of a poll defeat.
The current leader of the Lib Dems, Nick Clegg, has said he was not involved in any secret discussions.
Labour sources say talk of defections rather than discontent is overblown.
BBC political correspondent Iain Watson said it is known that some prominent Labour MPs are unhappy with the direction the party is taking.
It comes amid mounting criticism of the prime minister's performance on issues such as Gurkha settlement rights and MPs' expenses, with opposition parties claiming his authority has been fatally undermined.
When Lord Ashdown was leader of the Liberal Democrats he discussed forming a coalition with Labour under Tony Blair but the plans were never put into practice.
Now he has told the Daily Telegraph that the conditions could be in place for defections to the Lib Dems after the next election.
However, Lord Ashdown has kept the identities of his Labour confidants secret.
A spokeswoman for Mr Clegg told the BBC he was not involved in any secret discussions or secret deals with politicians from other parties.
But she said he was happy to work openly with MPs from all parties on issues of "moral importance" such as improving the rights of the Gurkhas.
This week MPs voted by 267 to 246 in favour of a Lib Dem motion offering all Gurkhas equal right of residence, with the Tories and 27 Labour rebels backing it.