Alastair Campbell has changed his mind and will now give evidence to an inquiry into whether the government exaggerated the threat posed by Saddam Hussein.
There has been a change of heart over Mr Campbell's appearance
The U-turn comes four days after Tony Blair's director of communications turned down an invitation to appear before MPs on the Foreign Affairs Select Committee inquiry.
Mr Campbell will face questions at 1500BST on Wednesday over his role in the production of a dossier which included a prominent claim that Iraq could launch a chemical or biological strike within 45 minutes.
Intelligence sources had told the BBC they were unhappy at the prominence given to this claim - with one source claiming Mr Campbell had asked for the dossier to be "sexed-up".
The main reason given for the UK taking part in military action was that Saddam Hussein presented a serious threat with his weapons of mass destruction.
Committee chairman Donald Anderson welcomed the news that Mr Campbell had changed his mind adding that it would be an "interesting session".
"He can help the committee in a number of key areas, including the drafting
of the two dossiers," he said.
Tory home affairs spokesman Oliver Letwin said he was pleased Mr Campbell was making himself available to the inquiry, but said it would not stop his party pushing for a full judicial inquiry.
And Liberal Democrat leader Charles Kennedy said: "I am pleased the government has finally seen sense and agreed that Alastair Campbell should give evidence into his handling of intelligence material in the run-up to the Iraq war."
Downing Street said Mr Campbell had always wanted to give evidence, but had decided not to because it was not normal practice for an official to appear before such an inquiry.
Short: Accused Mr Blair of a 'series of half truths'
The reason for the change of heart was an unspecified newspaper allegations at the weekend which Mr Campbell now wanted to rebut.
Downing Street said: "That prompted us to think again about the precedence issue and this is different in that Alastair was acting as chairman of a cross-departmental
communications committee and therefore the questioning falls into that category
rather than about personal advice he would have given to the prime minister."
Tory member of the foreign affairs committee John Maples said Mr Campbell's "fingerprints have been all over the so-called dodgy dossier for some time".
"Whatever he does he does with the prime minister's approval and authority," Mr Maples told BBC Radio 4's World at One programme.
"We are talking here about a decision for the country to go to war."
Mr Blair's official spokesman said Foreign Secretary Jack Straw would appear before the committee to answer policy questions.
Last week ex-cabinet minister Robin Cook accused Mr Blair's government of "not presenting the whole picture" in the run-up to war with Iraq during his appearance before the committee's inquiry.
He made the claim when he appeared as the first witness at the Commons inquiry into whether the threat posed by Saddam Hussein was exaggerated.
Fellow ex-minister Clare Short followed - accusing Mr Blair of a "series of half truths, exaggerations, reassurances that weren't the case" in the run-up to war.
She said she presumed Mr Blair saw the devices he used to get the UK to back America against Iraq as "honourable deception".
Mr Cook's evidence is likely to pour fuel on the row over WMD
Both ministers quit because of their disquiet over the government's policy on Iraq.
Mr Cook, an ex-foreign secretary, told the MPs he had "no doubt about the good faith of the prime minister", but said the "burning sincerity and conviction of those involved in exercise" was a "problem".
This conviction had led to intelligence material being carefully selected to back up their case for war - rather than being used as a basis for assessing whether or not Saddam posed a threat, he said.
A similar probe is being carried out by Congress in Washington into whether the Bush administration misread or inflated threats posed by Iraq before going to war.
Since the conflict was declared over, there have been no significant finds in the search for Saddam Hussein's WMD and in recent days there has also been a resurgence of military clashes involving US forces.
A separate inquiry by Parliament's intelligence and security committee, which meets in private, is also going to look at Iraq's WMD.