Mr Straw says separate evidence backs the UK claim
The UK Government has defended its claim that Iraq tried to buy uranium from Africa, despite the White House saying it was unfounded.
In a letter to a senior MP, Foreign Secretary Jack Straw said the UK had additional information to support the claim that Iraq had sought uranium from Niger, but this intelligence had not been passed on to the US administration.
In Washington, the CIA has accepted the blame for allowing the claim to be included in a speech by President George W Bush, even though the agency had long had doubts about its credibility.
The uranium claim was used by both governments to build a case for going to war over Iraq's alleged weapons of mass destruction.
Fresh doubts about intelligence information used by Britain to make the case for war have also been voiced by former chief UN weapons inspector Hans Blix.
No weapons of mass destruction have so far been found.
Mr Bush on Saturday tried to draw a line under the affair, which Democrats have seized on to challenge the president's authority.
A new opinion poll shows that public support for Mr Bush's handling of the conflict has fallen nine points to 58%.
The BBC's Nick Childs in Washington says though it's too early to say how significant the row will become, the administration is suddenly on the defensive.
The UK Government's insistence on standing by the Niger claim in spite of Washington's decision to back down has deepened confusion about the intelligence itself.
The claim was first made public in a dossier on Iraq released by the UK in September last year.
The claim was then cited in President Bush's State of the Union speech to Congress in January - in what the White House now says was a mistake.
It has emerged that long before that, in February 2002, Ambassador Joseph Wilson - now retired - was sent to Niger by the CIA to verify the claim. He reported it was unfounded.
Mr Straw on Saturday said his government had not been told of Mr Wilson's visit.
And he said separate intelligence which London had not passed to Washington in any case meant that the UK was still certain Iraq had indeed tried to get uranium in Niger.
In a letter to Donald Anderson MP, chair of the House of Commons Foreign Affairs Committee, Mr Straw wrote:
"The CIA expressed reservations to us about this element of the September dossier...
"However, the US comment was unsupported by explanation and UK officials were confident that the dossier's statement was based on reliable intelligence which we had not shared with the US... A judgement was therefore made to retain it."
The prime minister's office said the extra intelligence had come from a foreign service and could not be disclosed.
A spokesman said it was entirely different from the information the CIA had disavowed.
Mr Anderson has called on the government to reveal its source to clear up the confusion.
In his criticism, Hans Blix said the UK Government made a fundamental mistake when it declared that Saddam Hussein could deploy weapons of mass destruction in 45 minutes.
He told the British newspaper the Independent on Sunday that he believed the British Prime Minister, Tony Blair, overinterpreted the intelligence Britain had.
The row over intelligence which has been dominating British politics in recent weeks has now seized the US.
In his January address, Mr Bush said: "The British Government has learned that Saddam Hussein recently sought significant quantities of uranium from Africa".
CIA director George Tenet acknowledged that his organisation was wrong to let President Bush include the claim in his speech.
Mr Bush has given his support to Mr Tenet and said he now considers the matter closed.
However, our Washington correspondent says pressure may continue on Mr Tenet, whom some Republicans distrust for having been appointed by President Clinton.
And the Democrats also continue to press the president.
"This government either is inept or simply has not told us the truth," said Howard Dean, a contender for the Democrat presidential nomination.
An opinion poll for the Washington Post and ABC News on Saturday showed the country split on the issue, with 50% saying the president deliberately exaggerated evidence of Iraq's weapons of mass destruction and 46% saying he had not.
And the poll shows opinion becoming disillusioned with the Iraq conflict.
For the first time, more than half the respondents said there had been an unacceptable level of US casualties in Iraq.