TONY Blair’s personal credibility was called into question yesterday as a member of his Cabinet admitted the Prime Minister had "a particular problem" persuading the public to trust him.

Allies of Gordon Brown were talking openly about the make-up of the government and strategy when the Chancellor takes over, amid a growing sense of crisis surrounding the Blair administration.

Mr Blair, who is due to fly to Washington on Thursday on the first leg of a round-the-world diplomatic mission, also faced heightened pressure over the justification for war in Iraq.

Hans Blix, the United Nations chief weapons inspector, questioned Mr Blair’s claim that Saddam Hussein possessed weapons of mass destruction that could be fired within 45 minutes. Mr Blix described the allegation as "highly unlikely" and added: "I think that was a fundamental mistake. I don’t know exactly how they calculated this figure of 45 minutes in the dossier of September last year. That seems pretty far off the mark to me".

Mr Blix talked to the Prime Minister several times in the run-up to war and said Mr Blair was "strongly convinced" of the existence of weapons of mass destruction.

Mr Blair yesterday offered a bullish defence of his policy. He said: "Nobody was in any doubt of the threat posed by Saddam. ... When we see the Iraqi people making at last the first tentative steps towards self-government, and when the United Nations’ representative is already talking about 300,000 people in mass graves, then I hope that at least one thing that we can all agree on - the world is more secure, Iraq is a better place and will be a better place with Saddam Hussein out of power."

Mr Blair’s official visit to the United States could be overshadowed by the unprecedented fall-out between the CIA and MI6. The US intelligence agency has criticised the British secret service for supplying now- discredited information that Saddam had attempted to acquire uranium from Niger.

Downing Street played down the significance of the rift and said the government continued to stand by its dossier containing the 45-minute claim. A spokesman added that British intelligence on uranium was based on reports from Italy and that it was not in a position to give further details.

However, Robin Cook, who resigned from the Cabinet over the Iraq war, said: "It is time the government came clean and published the extra evidence they claim proves there was a uranium deal. If it was not good enough to be in the president’s address, it was not good enough to go in the Prime Minister’s dossier."

Peter Hain, now in Mr Cook’s old job as leader of the house, was one of a string of ministers who appeared on television yesterday in a bid to shore up Mr Blair’s position.

But the Leader of the House, who last month caused a major headache for Labour when he suggested the rich should pay more tax, made another gaffe, saying Mr Blair had "a particular problem" with trust.

Speaking on GMTV, he said: "For our government, and for Tony Blair specifically, as the head in this context, [trust] is a particular problem that we are suffering at the moment."

Mr Blair was defended by Peter Mandelson, the former minister, and Patricia Hewitt and Alistair Darling, both Cabinet members, but Clare Short, the former international development secretary who quit over the aftermath of the war, said he should stand down before things get "nastier". She added: "There is lots and lots of muttering going on because the trade unions are very unhappy, that partner of ours, and, of course, the degree of trust in the country has gone down remarkably."

Ms Short insisted she was not acting on behalf of anyone else, but said: "Obviously there’s Gordon Brown sitting there - and everybody knows he’s there - and he’s a big figure."

William Hill, the bookmakers, cut the odds on Mr Blair quitting by the end of the year from 20/1 to 6/1.