Scores of MPs are backing an early day motion demanding that the Government spell out its case after a minister admitted that an important claim about Saddam Hussein’s weapons was based on uncorroborated information.

Congress is also to investigate the Bush Administration’s assertions that Saddam held massive stockpiles of illegal weapons. Coalition Forces have so far found only two suspect lorries in spite of intensive searching for the arsenal, and even the US Defence Secretary Donald Rumsfeld has conceded that Saddam might have destroyed any weapons he had before the invasion.

Further doubts about the rationale for the war and its conduct were raised yesterday when an American team reported that the bunker bombed in an attempt to assassinate Saddam on the first night of the war did not exist. The head of the search operation was quoted as saying: “What they saw were giant holes, no underground facilities, no bodies.”

The questioning of the intelligence used to justify the war came as the Prime Minister admitted to British forces in Iraq that many thought he had been wrong to go to war.

Standing on the terrace of Saddam’s former summer palace in Basra, he lavished praise on the troops of the Seventh Armoured Brigade, saying: “I know there are a lot of disagreements in the country about the wisdom of my decision to order the action, but I can assure you of one thing: there is absolutely no dispute at all about your professionalism and your courage and your dedication.”

Mr Blair made no reference to the elusive weapons, but referred constantly to the liberation of Iraq, adding to the impression that both the British and American Governments had decided to make that, rather than the weapons, the justification for war.

Helping to liberate the Iraqis was a “huge, momentous and mighty act”, he said. “When people look back they will see the war as one of the defining moments of the century,” he said.

In London, however, the concern about the justification for the war was growing and it increased when Adam Ingram, a Defence Minister, said that Mr Blair’s assertion that Saddam would be able to launch biological weapons within 45 minutes had been based on only one source.

Downing Street also faced allegations — which it denied — that it had put pressure on the intelligence services to toughen up their weapons dossier.

In Washington both Democrats and Republicans were also expressing fears that the main premises on which the war was justified — the weapons and links with al-Qaeda — were based on false or exaggerated intelligence. There were also suggestions that intelligence information may have been manipulated to rally public opinion behind the war. Both the Senate and House of Representatives are to hold hearings to determine whether “the analysis relayed to our policy-makers was accurate and unbiased”.

Jane Harman, the senior Democrat on the House Select Committee on Intelligence, said: “This could be the greatest intelligence hoax of all time. I doubt it, but we have to ask. It was the moral justification for the war. I think the world is owed an accounting.” The House committee has written to George Tenet, the CIA Director, asking him to respond by July 1 on several questions, with a view to holding hearings that month.

A copy of the letter, seen by The Times, asks Mr Tenet whether the intelligence was of sufficient quantity, quality and reliability, how it was analysed, and whether “any dissenting views were properly weighed”. The Senate Intelligence Committee is also planning similar hearings.

Porter Goss, the House committee’s Republican chairman, told The Times: “My concern is that we did not have enough good intelligence to draw the necessary conclusions that our policy-makers need to be completely confident. Wouldn’t it be nice if we gave them better information to base their judgments on?” The CIA has launched a separate review, which was originally suggested last October, to monitor the intelligence process that led up to the war. The main call for that review came from Donald Rumsfeld, the Defence Secretary who was then frustrated by the CIA’s inability to find a “smoking gun” that would justify an invasion.

In an effort to unearth incriminating intelligence against Saddam, Mr Rumsfeld created the Office of Special Plans, a unit inside the Pentagon under the direction of his hawkish deputy Paul Wolfowitz. That became a direct rival not only of the CIA, but the Pentagon’s Defence Intelligence Agency, and its staff relied heavily on information from Ahmed Chalabi, the exiled leader of the Iraqi National Congress and the Washington hawks’ choice as the man to lead a post-Saddam Iraq.

Patrick Lang, a former director of Middle East analysis at the Defence Intelligence Agency, said this month that the Office of Special Plans “started picking out things that supported their thesis and stringing them into arguments that they could use with the President. It’s not intelligence. It’s political propaganda.”

The Pentagon strongly denies that it manipulated evidence or relied too heavily on untested sources. But defence officials concede that Saddam’s illegal weapons may no longer be in Iraq.

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