Lord Ashdown of Norton-sub-Hamdon revealed yesterday that he had turned down an invitation from Gordon Brown to sit in his first Cabinet, in effect killing the last remaining prospect of a Liberal Democrat minister serving in the new administration.

Mr Brown offered the job of Northern Ireland Secretary to the former Lib Dem leader behind the back of Sir Menzies Campbell, the present leader. Amid much confusion, each side blamed the other for leaking details of the offer.

The move, although rejected, will sour relations between Mr Brown and Sir Menzies, who both sit for constituencies in Fife and are personal friends. Senior Lib Dems accused Mr Brown of a breach of trust.

Both faced criticism from their own parties last night and aides to Sir Menzies said that it had raised questions about his leadership. Allies of Mr Brown said that he was unlikely to go ahead with plans to offer Liberal Democrat peers ministerial jobs since it was clear that they would have to resign from the party to serve.

The Tories were delighted at what they said was a “giant snub” to Mr Brown. William Hague, the Shadow Foreign Secretary, said: “It is a very incompetent start . . . Gordon Brown has had years to plan this, and the first thing he does is a disaster.”

Mr Brown was criticised for the Ashdown job offer by some on the left of the party. Peter Kilfoyle, a former minister, said: “It is something that upsets me and many of my colleagues . . . In the north of England, the Lib Dems are the main political opposition and this doesn’t help one iota.”

John McDonnell, who attempted to challenge Mr Brown for the leadership, said: “Gordon may have mentioned wanting a government of all the talents but at no stage in the hustings for his election as leader did he suggest a coalition with the Liberals.”

Initially Mr Brown informed Sir Menzies that he wanted to approach Lord Ashdown and another Lib Dem peer, the party’s health spokeswoman, Baroness Neuberger, with a view to offering them posts in his Government, although at that stage not in the Cabinet.

The episode risked damaging the perception of the Liberal Democrats as an independent political party while Mr Brown’s move, although unsuccessful, suggested he is taking a radical approach as he puts together his first Government.

Lord Ashdown told the BBC: “I told him [Gordon Brown] 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.”

Mr Brown, who plans a programme of constitutional reform, still intends to bring in people from outside Parliament to play a role in his administration, some of whom will be appointed peers, allies say.

Such a “big tent” approach to Government would present a challenge to rival parties, although the Conservatives said that they would consider any genuine offer for cross-party collaboration on its merits.

Lady Neuberger declined to comment, although friends said that she had not received any approach from Mr Brown or on his behalf.

Another Lib Dem peer, Lord Lester of Herne Hill, a human rights lawyer with a long interest in constitutional reform, expressed enthusiasm for Mr Brown’s plans for a Bill of rights, work towards a written constitution and reform of the House of Lords but ruled out breaking ranks with his own party to join Labour.

David Liddington, the Shadow Northern Ireland Secretary, said that it was “utterly humiliating” for Peter Hain, who holds the portfolio.

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