The bureau of Intelligence and Research (INR), the State Department's in-house analysis unit, and nuclear experts at the Department of Energy are understood to have explicitly warned Mr Powell during the preparation of his speech that the evidence was questionable.

In the presentation, in which the US laid out its case for a pre-emptive war on Iraq, Mr Powell accused Iraq of importing special aluminium tubes as evidence that Baghdad was still working on a programme to produce atomic weapons.

But the INR disputed claims by the CIA and the Pentagon that the tubes were intended for a nuclear weapons programme. While the INR and the Energy Department had already made their opposition known in a footnote to the top-secret National Intelligence Estimate prepared in October 2002, the bureau is understood to have again told Mr Powell during the preparation of his February speech that its analysts were not persuaded that the tubes could be used in centrifuges to enrich uranium.

In the controversy over pre-war intelligence on Iraq, both the White House and the Central Intelligence Agency have taken blame for questionable claims made by President George W. Bush.

But Colin Powell, the US secretary of state, has escaped such scrutiny despite having delivered a critical February 5 presentation to the United Nations that ignored strong doubts within the US government about the truth of one of the central elements of its claim that Iraq was reconstituting a nuclear weapons programme.

While most of the attention has focused on Mr Bush's now-discredited claim that Iraq was attempting to buy uranium from Africa, Mr Powell's claims about the tubes have not held up much better under scrutiny.

Iraq, he alleged, had attempted to buy the tubes from 11 countries - efforts that continued last year even after weapons inspectors arrived in Baghdad. Mr Powell acknowledged there were "differences of opinion" among experts in the US government on their intended use but some insisted they were of a quality far higher than the US used in artillery rockets, the use the Iraqi government had claimed for them.

Inspectors of the International Atomic Energy Agency said in March, however, that it was "highly unlikely that Iraq could have achieved the considerable redesign to use them in a revived centrifuge programme", the use for which Mr Powell argued they were intended.