The prime minister will face a leadership challenge if he goes to war without a second United Nations resolution, it was claimed on Tuesday.

Father of the House Tam Dalyell said Labour's rank-and-file would attempt to unseat the prime minister if he backed a US-led offensive.

Speaking as hopes of a second resolution receded, Dalyell said Blair was living in dangerous times.

"As soon as it becomes clear that the UN is disregarded, yes, certainly a letter will go out to our colleagues asking for a special conference of the Labour Party," he told the Today programme.

"I don't think it is possible to exaggerate the degree of concern about the illegality of what is proposed."

Labour Party chairman John Reid denied suggestions that the prime minister would face a challenge.

"Why don't we spend our time discussing in the party what is the really important issue, which is not getting rid of Tony Blair, it is getting rid of the weapons of mass destruction of Saddam Hussein," he said.

The warning came after Clare Short accused Blair of recklessness on Iraq.

The international development secretary warned she would quit unless the UK secured a second resolution.

"If there is not UN authority for military action or if there is not UN authority for the reconstruction of the country, I will not uphold a breach of international law or this undermining of the UN and I will resign from the government," she told the BBC on Sunday night.

"I think it's time for cards on the table. People are speculating and making all sorts of statements about my intentions.

"I think I owe it to my colleagues in the government and members of the Labour Party to just be truthful about my position. It's the time to say what my intentions are."

On Monday Downing Street expressed its "surprise" at Short's broadside against the prime minister.

The prime minister's spokesman has again today refused to say whether the international development secretary enjoys the confidence of her boss.

The decision to sidestep the question fuelled further speculation that Blair is ready to axe the Cabinet minister once the current crisis is resolved.