Tony Blair was under mounting pressure yesterday after he refused to withdraw discredited claims by the secret intelligence service MI6 that Saddam Hussein tried to buy uranium to make nuclear weapons.
The controversy over documents supplied by MI6 and exposed as crude forgeries by the International Atomic Energy Agency (IAEA) before the war in Iraq now threatens to erupt into a full-blown political scandal on both sides of the Atlantic.
Yesterday the Prime Minister stood by the dossier on Saddam's alleged weapons of mass destruction (WMD) issued by the Government last September, which included the claim that Iraq had "sought the supply of significant quantities of uranium from Africa" even though it had no active civil nuclear power programme.
On the same day the Foreign Affairs Committee in the US Congress was given secret testimony that Niger had provided Iraq with 500 tons of uranium oxide for its secret nuclear bomb programme.
The Government's dossier on WMD also contained Mr Blair's assertion that Iraq was able to launch chemical and biological weapons within 45 minutes, which is now widely discredited.
The threat of Saddam acquiring nuclear weapons became a linchpin in both governments' campaigns to build a case for war. But the allegation was blown apart in March by the IAEA, after a cursory investigation.
Yesterday Robin Cook, the former foreign secretary, urged Mr Blair to withdraw his claim in the Commons last September that Saddam was "actively trying to acquire nuclear weapons capability".
The Prime Minister said at the time: "We know that Saddam has been trying to buy significant quantities of uranium from Africa, although we do not know whether he has been successful."
Mr Cook, who resigned from the Cabinet over the Iraq war, challenged Mr Blair in the Commons, asking whether he had been advised that the documents on which the claim was based were forged. He asked Mr Blair to correct the record now by saying that "he regrets in all good faith he gave the House information which has since turned out to be wrong".
Mr Blair refused to do so, insisting there was intelligence to back up the claim. He said: " I'm not going into the details of what particular intelligence it was. But there was intelligence judged by the Joint Intelligence Committee at the time to be correct." He said the Government was not in a position "to say whether that is so or not" until after the investigation to be carried out by Parliament's Intelligence and Security Committee.
When the Prime Minister is quizzed by the committee, he is expected to say that the Government had more than one source for the allegation. One British official said: "There were a number of sources for the text in our dossier on that and we stand by it."
Last night the IAEA expressed surprise that Mr Blair did not take the opportunity offered by Mr Cook to abandon the allegation. Melissa Fleming, a spokeswoman for the IAEA, said: "These were blatant forgeries. We were able to determine that they were forgeries very quickly."
It only became apparent to the rest of the world in March that the basis of the allegation, letters purportedly exchanged between Iraqi agents and the government of Iraq, had been faked. What is embarrassing is how rudimentary the forgeries turned out to be. One such letter, for example, had apparently been signed by a Niger foreign minister who, at the time of its signing, had in fact been out of office for more than a decade.
The letters contained several other basic errors that were immediately picked up by the IAEA investigators.
Members of Congress are now turning up the heat on the White House to explain why intelligence that even the CIA was dubious about was included in President George Bush's state of the union address in January. Some members have said it was the only reason they supported the war.
The implication that Mr Bush may have deliberately misled Congress, and the world, in that speech was made in a private letter sent this week from Henry Waxman, a leading Democrat in the House of Representatives, to Mr Bush two days ago. The letter was seen yesterday by The Independent. He wrote: "I urge you to explain why you cited forged evidence about Iraq's efforts to obtain nuclear materials in your state of the union address.
"That a President could cite forged evidence in such an address - on a matter as momentous as impending war -should be unthinkable."
"Using little more than a Google search, IAEA experts discovered indications that should have been evident to novice intelligence officials."
The issue is expected to be investigated closely by two congressional inquiries to be launched in the United States.
In his state of the union address, Mr Bush said: " The British Government has learned that Saddam Hussein recently sought significant quantities of uranium from Africa."
In April, a statement posted on the White House website said: "He [Saddam] recently sought significant quantities of uranium from Africa, according to the British Government."
Mr Waxman said last night: "The United States knew that information independent of Tony Blair to be a hoax, to be incorrect. At least our CIA knew it. Maybe the President was relying on Tony Blair to make a statement and therefore the President's statement was accurate.
But it's quite a deceptive way to make a presentation. Of course it leaves open the question: what did Tony Blair know about this?"
The US Secretary of State Colin Powell, who spent four days ensconced with the CIA reviewing evidence before addressing the UN, opted to drop the uranium allegation from his testimony at the last minute.