The government is under fire over claims it failed to inform last year's Hutton inquiry that key Iraq weapons intelligence was flawed.
By Sean Coughlan and Brian Wheeler
BBC News Online
Lord Butler's report has sparked fresh questions about the Iraq dossier
Downing Street says the piece of intelligence in question fell outside Lord Hutton's remit.
And - in any case - Tony Blair did not know it had been discredited when he gave evidence - even though MI6 had taken what is regarded as the unusual step of withdrawing the intelligence as "unreliable".
Mr Blair only learned that detail more recently "as a result of the Butler inquiry", the prime minister's official spokesman said.
The revelations - which emerged after journalists pored over Butler's findings - have added to pressure for a further inquiry into the political decisions that led up to war.
So how much do we know about the piece of intelligence that has sparked this latest row?
The first thing to point out is that it has nothing to do with the now infamous claim that Iraq could launch weapons of mass destruction within 45 minutes.
It was, rather, concerned with Iraq's alleged production of chemical and biological agents.
Nevertheless, Lord Butler said the information had a "major effect" on the government's September 2002 dossier on Iraq's weapons of mass destruction (WMD).
It was certainly believed at the time of writing.
But on 17 July 2003 - less than a month before the start of Lord Hutton's inquiry - MI6 withdrew the intelligence as its source was no longer considered reliable.
The Butler report says the false intelligence appear to confirm suspicions about Iraq
How much do we know about the source?
In the run up to the September dossier, Lord Butler says, several sources, which were later proved to be reliable, were providing information about Iraq's weapons - and these established sources tended to "present a less worrying view".
But between the production of the joint intelligence committee's (JIC) assessment of the threat from Iraq - and the publication of the government's public dossier another source - described as being "on trial" - emerged.
Butler says the impact of this new source was to "provide significant assurance to those drafting the government's dossier that active, current production of chemical and biological agent was taking place".
Information from this source "had a major effect on the certainty of statements in the government's dossier of September 2002 that Iraq possessed and was producing chemical and biological weapons," Butler says.
But after the war, doubts were cast on the accuracy of this source - and by July 2003, the Butler report says that the "sourcing chain had been discredited". As a consequence, two intelligence reports from this source were withdrawn.
A few weeks later, on Monday, 11 August 2003, the Hutton Inquiry began taking evidence - including from Tony Blair.
But the prime minister's spokesman says that when Mr Blair gave his evidence in August he was unaware that intelligence which had influenced the Iraq dossier had subsequently been discredited.
The prime minister was not aware of the withdrawal of faulty intelligence until it had been highlighted by the Butler Report, said his official spokesman.
But the spokesman added that this piece of false intelligence had only been "one part of the picture on chemical and biological weapons production".
The Butler report also repeatedly highlights the difficulties faced by the intelligence services and the government in drawing firm conclusions from shifting and fragmented pieces of information.
The Butler report says that although intelligence was "correctly reported" in the JIC assessment, by the time it had been "translated" into the September 2002 dossier the limitations of that intelligence were no longer clear.
This apparent certainty "may have left readers with the impression that there was fuller and firmer intelligence behind the judgements than was the case".
It also says there is still no final conclusion about what weaponry Iraq had before the war - with the report saying it would be a "premature to reach conclusions about Iraq's prohibited weapons".
"We believe that it would be a rash person who asserted at this stage that evidence of Iraqi possession of stocks of biological and chemical agents, or even of banned missiles, does not exist or will never be found."