In a remarkable letter released last night, the Foreign Secretary, Jack Straw, reveals a catalogue of disputes between the two countries, lending more ammunition to critics of the war and exerting fresh pressure on the Prime Minister.

The letter to the Foreign Affairs Committee, which investigated the case for war against Iraq, reveals that Britain ignored a request from the CIA to remove claims that Saddam was trying to buy nuclear material from Niger, despite concerns that the allegations were bogus. It also details a government decision to block information going to the CIA because it was too sensitive.

As diplomatic relations between America and Britain become increasingly strained over Iraq's WMD, Straw said that the Government had separate evidence of the Niger link, which it has not shared with the US.

The revelations come just four days before Tony Blair travels to America for his toughest visit there since he came to power in 1997. As well as WMD, the Prime Minister will also raise Britain's 'serious concerns' over the treatment of British citizens held at Guantanamo Bay.

Straw's letter reveals:

· That evidence given to the CIA by the former US ambassador to Gabon, Joseph Wilson - that Niger officials had denied any link - was never shared with the British.

· That Foreign Office officials were left to read reports of Wilson's findings in the press only days before they were raised as part of the committee's inquiry into the war.

· That when the CIA, having seen a draft of the September dossier on Iraq's WMD, demanded that the Niger claim be removed, it was ignored because the agency did not back it up with 'any explanation'.

Although publicly the two governments are trying to maintain a united front, the admission two days ago by the head of the CIA, George Tenet, that President Bush should never have made the claim about the Niger connection to Iraq, has left British officials exposed.

Last night, Downing Street and Foreign Office sources said that 'they would not blink' over the Niger claims. One Downing Street figure said that they were based on intelligence from a third country that was reliable. 'We are not backing down,' he said.

Another official said that the claim was based on the 'intelligence assessment' made at the time, leaving the door open to a climbdown if the intelligence is found to be wrong.

'I want to make it clear that neither I nor, to the best of my knowledge, any UK officials were aware of Ambassador Wilson's visit until reference first appeared in the press,' Straw said in the letter.

'The media has reported that the CIA expressed reservations to us about this element [the Niger connection] of the September dossier. This is correct. However, the US comment was unsupported by explanation and UK officials were confident that the dossier's statement was based on reliable intelligence which had not been shared with the US. A judgment was therefore made to retain it.'

Straw said that the Joint Intelligence Committee's assessment of the Iraqi nuclear threat did not just rest on attempts to procure uranium. There was also other evidence of links between the two countries and attempts to sign export deals.

Robin Cook, the former Foreign Secretary who has become a trenchant critic of the Government's case for war against Iraq, said that it 'stretched credibility' to say that the Americans and the British had failed to share such basic information.

'From all I know of the intimate relationship between the CIA and the Secret Intelligence Services, I find it hard to credit that there was such a breakdown of communication between them,' Cook said.

'It is time the Government came clean and published the evidence. The longer it delays, the greater the suspicion will become that it didn't really believe it itself.

'There is one simple question it must answer. Why did its evidence of the uranium deal not convince the CIA? 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.'

Yesterday, in another damaging broadside, Richard Butler, who was executive chairman of the United Nations Special Commission to Iraq from 1997 to 1999, said that anyone who had claimed that there was a link between Niger and Iraq should resign.

Referring to Australian politicians who had made similar claims, only to withdraw them and apologise later, Butler said: 'In the justification for the war, these claims were false and known to be false.

'A Minister who misleads Parliament must accept responsibility for it and resign. Ministers must be held responsible, not public servants.'

Special reports
Iraq: Observer special
Observer Iraq comment
Iraq: more from Guardian Unlimited

Iraq aftermath: The Niger row
13.07.2003: Blair ignored CIA weapons warning
13.07.2003: The Niger connection
13.07.2003: Even in Africa, Bush can't avoid Iraq



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Comment
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13.07.2003: Harry Barnes: Time to take the hawks at their word
13.07.2003: Letters: The truth on Blair
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06.07.2003: Ed Vulliamy on the lives of Iraqi civilians (part two)

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06.07.2003: Andrew Rawnsley: The truth will be elusive
06.07.2003: Menzies Campbell: Why did we really go to war?
29.06.2003: David Aaronovitch: Source of discontent
22.06.2003: Peter Beaumont: Dangerous liaisons
29.06.2003: Benjamin Barber: No country can democratise another
29.06.2003: Peter Preston: Campbell's day of reckoning
22.06.2003: Peter Beaumont: Secret squirrelling
08.06.2003: Anthony Sampson: Hijacked by that Mob at No 10
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08.06.2003: Focus: Were they weapons of self-delusion?
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08.06.2003: Euan Ferguson: A latte - and a rifle to go