A White House admission that a claim that Iraq had tried to get uranium in Africa was wrong has landed President Bush in a political row at home and has undermined a key part of the British government's dossier on Iraq. It has also opened up a split with Mr Bush's closest ally Tony Blair, who is standing by the claim.
It was one of the most startling lines in the British government paper on Iraq issued last September: "There is intelligence that Iraq has sought the supply of significant amounts of uranium from Africa."
Since Iraq had no civilian nuclear programme, the implication was clear - it was trying to make a nuclear bomb.
President Bush repeated the charge in his State of the Union address to Congress in January, though he sourced it to the British.
"The British Government has learned that Saddam Hussein recently sought significant quantities of uranium from Africa."
Not long afterwards, doubts about this allegation began to emerge.
The UN's nuclear watchdog, the International Atomic Energy Agency, said the claim was based on documents which had been forged.
The documents were supposed to be faxes of agreements between Niger and Iraq. Who forged them is not known.
Secret Niger mission
Now it has come out that nearly a year before the State of the Union speech, the US Government had sent a senior former diplomat, Joseph C Wilson IV, to Niger to make on-the-spot inquiries.
Mr Wilson broke cover this week to reveal that he had told the US Government that "it was highly doubtful that any such transaction had taken place."
Niger's uranium mines, he said, were under strict international control.
He further concluded, in an article in the New York Times: "I have little choice but to conclude that some intelligence was twisted to exaggerate the threat."
The question that arises of course was why the president mentioned the claim in the light of Mr Wilson's report.
The CIA has told the BBC that it passed on Mr Wilson's information through normal channels, which include the White House.
Indeed Mr Wilson himself said he had been asked to go to Niger because of a request by Vice President Dick Cheney's office.
So he presumed that Mr Cheney's officials had been informed of his findings.
It is interesting to note that the CIA, in its own assessment of Iraq's weapons issued shortly after that of the British government, did not mention the Niger connection.
Democrats in Washington are now demanding an inquiry. Had the president been kept in the dark or had the Wilson report got lost in the system and if so, why?
Senator Ted Kennedy said: "It's bad enough that such a glaring blunder became part of the president's case for war. It's far worse if the case for war was made by deliberate deception."
London and Washington split
President Bush, asked about the Niger issue at a news conference during his visit to South Africa, did not answer directly but said that he was "certain that Saddam Hussein had a weapons of mass destruction programme."
Like Mr Blair, he has dropped the assertion that Iraq actually had weapons. Both now say that it had a "programme."
Another question needs to be answered. Was the British Government informed of the Wilson report?
The British Foreign Secretary Jack Straw was asked about this by the House of Commons Foreign Affairs Committee and said he would "find out."
Soldiers are yet to find weapons of mass destruction in Iraq
The split with Mr Blair arises because the British Government is standing by its document.
It is doing so despite the White House admission, the Wilson report and the IAEA revelations because it says that it is relying on other intelligence.
The British Government also says that, when it drew up its dossier, it had not even seen the documents later shown to be forged.
Mr Blair told a committee of MPs on Tuesday that the evidence about the Niger link "did not come from these so-called forged documents. They came from separate intelligence."
The allegations, he said, were not "fantasy." He pointed out that in the 1980s, Iraq had bought uranium from Niger.
The Foreign Affairs committee asked what this other intelligence was but has not been told.
It wants to know and, in any case, has said that the language used in the dossier should have been "qualified to reflect the uncertainty."
Perhaps Mr Straw will tell the Intelligence and Security Committee which is also inquiring into Iraq and which works in secret.