Mandelson spat takes the shine off Labour's party

Labour celebrations on the eve of its first 100 days in power were overshadowed by questions about Peter Mandelson running the government while Tony Blair is on holiday in Tuscany.

Mr Mandelson, the minister without portfolio, came out fighting when he was questioned about his role on the World at One on BBC radio 4, which led to "dozens of listeners" telephoning the BBC to complain about his "overbearing, arrogant, pompous" performance. "We always get a steady stream of callers, but there was a marked number about his interview," said a BBC source.

Mr Mandelson earlier attacked "vain-glorious self-indulgent questions" about the prominent role he has played since Mr Blair left to spend his summer holidays in the villa near Siena owned by Geoffrey Robinson, the millionaire Paymaster General.

Although he is not a member of the Cabinet, Mr Mandelson has been in charge of handling the Government's public relations since the news broke last weekend about Robin Cook's affair with his secretary. Mr Mandelson spoke for the Government on the future of the Royal Yacht, the MI6 inquiry into Chris Patten, and chaired a press conference for Lord Simon, the trade minister, to announce he was selling his pounds 2.3m BP shares.

When it was suggested on BBC radio that he was becoming the "face of the Government", Mr Mandelson retorted: "There you go again," he said. "You can't get off the subject."

Questions about Mr Mandelson's role also are causing friction between the Government and the Liberal Democrats since Mr Blair gave Paddy Ashdown and key lieutenants seats on a special Cabinet committee.

Mr Mandelson, an architect of the links, warned the Liberal Democrats at the press conference they should decide whether they were going to exercise "a little responsibility" after being brought into the Government by Mr Blair. "I think the jury is out about what they are going to do."

That was seen as a threat to the Liberal Democrats to toe the Government line. Government sources said Mr Mandelson was aware of split in the Liberal Democrats about whether it was a good or bad thing to join the Government on the committee, and he was developing that debate.

But Liberal Democrat sources said it was retaliation against Norman Baker, a Liberal Democrat MP who has raised a series of Commons questions about Mr Mandelson's role in the Government. Mr Mandelson was said to be "deeply irritated" last weekend when Mr Baker's questions led to reports that Mr Mandelson had not spoken in the Commons for a year.

"He is their political warfare minister and he is getting his retaliation in.

"This goes back to Parliament and the hundreds of questions that Norman Baker put down, getting him to spell out that he hasn't spoken in the House since mid-1996. He was very deeply irritated about that," a Liberal Democrat source said.