Gordon Brown has been accused of "dirty tricks" by Lib Dems angry at his attempt to recruit their ex-leader Paddy Ashdown to his first Cabinet.
Lord Ashdown is refusing to comment further on the offer
Lord Ashdown rejected an approach by the prime minister-in-waiting to serve as Northern Ireland Secretary.
Lib Dem sources said Mr Brown had used "underhand tactics" by going behind the back of leader Sir Menzies Campbell.
But junior minister James Purnell insisted it was a genuine offer and Mr Brown wanted "a new kind of politics".
Voters wanted Mr Brown to be open to new ideas, he told BBC News, and "it would be very odd to say that we're not going to look at those ideas, we're not going to use your talent - just because you happen to have a different party political label".
Asked if he thought Mr Brown was trying to undermine his party, Sir Menzies, who describes the chancellor as a good friend, said he was not "in the business of attaching blame to others one way or another".
But sources close to the Lib Dem leader accused Mr Brown of "trying to stitch us up... dump on us... divide us".
They said Mr Brown's backers had deliberately gone behind Sir Menzies' back in an effort to "pick off" Lord Ashdown.
Lib Dem MP Norman Baker said: "It looks like an attempt to destabilise us.
"If he was serious about making an offer, he would have gone about it in a different way."
He added: "If he assumes that Liberal Democrats will want to join Labour after the next election he has miscalculated rather badly."
Mr Brown's move was also met with anger on the left of the Labour Party, with former leadership hopeful John McDonnell saying "at no stage in the hustings for his election as leader of the party did he ever suggest a coalition with the Liberals".
Shadow Foreign Secretary William Hague, for the Conservatives, said there was nothing wrong with "trying to reach agreement across politics".
But he added: "I think this was a rather naive and bungled attempt to do so."
Lord Ashdown was offered the job on Wednesday after Sir Menzies had already rejected the idea of Lib Dems joining a Brown Cabinet.
In a statement to BBC Radio 4's Today programme, the Lib Dem peer said: "I told him that I could not conceivably consider such a position unless my leader told me that he thought it was a good idea and even if he did, I didn't."
He added: "You do not build partnership government by seeking to add the Liberal Democrats as a bungalow annexe to a Labour government."
He later refused to comment further on the offer.
It has been widely reported that Lord Ashdown and Sir Menzies Campbell were close to joining Tony Blair's first Cabinet when he came to power in 1997.