Former UN chief weapons inspector Hans Blix accused the British Government of using spin in its controversial dossier on Iraq's weapons of mass destruction.
Dr Blix has previously criticised Mr Blair over Iraq
Dr Blix criticised the "culture of spin, of hyping" and told the BBC Radio 4's Today programme that he hoped governments would be more cautious in the future use of special intelligence.
He compared the way Britain and America were sure Iraq had weapons of mass destruction programmes to the way people in the Middle Ages were convinced witches existed and so found them when they looked.
In response, the British Foreign Office said Saddam Hussein's possession of weapons of mass destruction was a matter of fact and the search for them would continue.
Dr Blix's comments come amid the Hutton inquiry into the death of British government scientist Dr David Kelly, who apparently killed himself after he was named as the source for a BBC story that the government "sexed up" the dossier.
Dr Blix said: "The UK paper that came out in September last year with the famous words about the 45 minutes - when you read the text exactly I get the impression it wants to convey to the reader and lead the reader to conclusions that are a little further reaching than the text needs to mean.
"One can read it restrictively but one can also lead to far-reaching conclusions and I think many people did."
Dr Blix argued that exaggeration, spin and hype damaged government credibility.
"We know that the advertisers will advertise a refrigerator in terms they do not quite believe in but you expect governments to be more serious and have more credibility," he said.
Dr Blix said he understood that information had to be simplified but people still expected it to be reliable.
He accused the British and American governments of "over-interpreting" intelligence.
"They were convinced that Saddam was going in this direction and I think it is understandable against the background of the man," he said.
"But in the Middle Ages people were convinced there were witches. They looked for them and they certainly found them.
"This is a bit risky. I think we were more judicious, saying we want to have real evidence."
Dr Blix said the coalition could have waited and continued with UN inspections for a few more months, but they did not have the patience to do so.
Now though, he said, US and British inspectors were free to go anywhere in Iraq, but were calling for the patience for themselves which they had failed to give to the UN.
Dr Blix had previously criticised Prime Minister Tony Blair for making a "fundamental mistake" in claiming that Saddam Hussein could deploy weapons of mass destruction in 45 minutes.
And his latest comments come a day after he said Iraq probably destroyed all of its weapons of mass destruction more than a decade ago.
Responding to Dr Blix's criticisms, a Foreign Office spokesman said:
"Saddam's possession of weapons of mass destruction is a matter of fact.
"Successive UN Security Council resolutions concluded not only that he had them but also had used them against his own people. Dr Blix's own 173 page report set out in great detail Saddam's history of obstruction of the UN inspectors.
"The process of searching for weapons of mass destruction is continuing. It will be thorough and deliberate, despite the difficult security environment."
The spokesman stressed that Parliament's Intelligence and Security Committee had concluded there was convincing intelligence that Iraq had active chemical, biological and nuclear programmes and the capability to produce chemical and biological weapons, as well as continuing to develop ballistic weapons.
Conservative shadow foreign secretary Michael Ancram said a judicial inquiry into the run-up to the Iraq, not just into the circumstances of Dr Kelly's death, was now urgently needed.
"Dr Blix's comments raise serious questions to which the government must now respond," said Mr Ancram.
Liberal Democrat MP Menzies Campbell said Dr Blix's remarks reinforced the need for the UK Government to publish the full legal advice it received on the eve-of-war.
"Dr Blix's careful academic analysis has dealt yet another damaging blow to the British government's case for war," said Mr Campbell.