A senior ex-intelligence official has disputed Alastair Campbell's evidence about a dossier which outlined the threat posed by Saddam Hussein.
Tony Blair's ex-spokesman told the Iraq inquiry last year the September 2002 document was designed to set out UK concerns, not "make the case for war".
But Michael Laurie said those producing it "saw it exactly as that and that was the direction we were given".
Mr Campbell said he had "nothing to add" to the evidence he had given.
The Iraq report is due to be published later this year and the inquiry panel, headed by Sir John Chilcot, is currently considering all the material it has received - including de-classified documents and evidence given during more than 100 public hearings over the past 18 months.
Among fresh documents it published on Thursday was a letter from Major General Michael Laurie, a member of the Defence Intelligence Staff in the run-up to the March 2003 invasion.
'Defended every word'
In the letter, written in response to evidence given by Mr Campbell in January 2010, he disputes Mr Campbell's explanation for the motivation behind the dossier on Iraq's alleged weapons of mass destruction (WMD).
As No 10 director of communications between 1997 and 2003, Mr Campbell played a key role in the dossier - which contained the controversial claim that WMD could be deployed in 45 minutes of an order to use them.
In his evidence last year, Mr Campbell said he "defended every single word" of the document and that it did not "in any sense misrepresent the situation" with regard to Iraq at the time.
Describing it as a "cautious" assessment, he insisted it had not been designed to present the "case for war" but to highlight why Mr Blair was increasingly concerned about the threat posed by Iraq.
But Mr Laurie - who was responsible for delivering intelligence material on Iraq to assessment teams within the Ministry of Defence - said he disagreed with Mr Campbell's argument.
"Alastair Campbell said to the inquiry that the purpose of the dossier was not to "make a case for war," he wrote. "I had no doubt at this time this was exactly its purpose and these very words were used."
He added: "We knew at the time that the purpose of the dossier was precisely to make a case for war rather than setting out the available intelligence."
A similar document produced six months earlier had been rejected as it had "not made a strong enough case", he claimed.
"From then until September (2002) we were under pressure to find intelligence that could reinforce the case."
'Direction and pressure'
But despite what he said was probably the most thorough "scrutiny of every piece of ground" in Iraq, he said intelligence experts could find no evidence of planes, missiles or any equipment related to WMD.
Mr Laurie - who is now chief executive of the charity Crimestoppers - also suggested the Joint Intelligence Committee ultimately responsible for producing and signing off the dossier had come under "direction and pressure" - something Mr Campbell has always strongly disputed.
"During the drafting of the final dossier, every fact was managed to make it as strong as possible, the final statements reaching beyond the conclusions intelligence assessments would normally draw from such facts," he added.
The dossier included a foreword by Mr Blair in which he wrote that he believed the intelligence had established "beyond doubt" that Iraq continued to produce chemical weapons.
Mr Campbell, who drafted the first version of the foreword - ultimately approved by Mr Blair - said no-one in intelligence challenged this statement.
However, on the 45-minute claim - which was retracted after the war - he has said the dossier "obviously" could have been clearer about it referring to battlefield munitions.
Questions about Mr Campbell's role in the dossier were at the centre of a post-war row with the BBC culminating in the death of the government weapons expert Dr David Kelly and the subsequent Hutton inquiry.
Former Lib Dem leader Sir Menzies Campbell said it would be up to the Chilcot inquiry to resolve what he said was "a direct and unequivocal conflict of evidence" about the dossier.
"The controversy surrounding the dossier of September 2002 lies right at the very heart of the criticisms made of the Blair government," he said.
"Sir John Chilcot and his colleagues will have to decide which of these two versions is correct. They can't both be right."
The BBC News Channel's chief political correspondent Laura Kuenssberg said Mr Campbell had said he had "nothing to add" to evidence he had given to the inquiry.
He said the dossier was not about making the case for war but had set out why the government had become more concerned about Iraq's weapons. He added he had "never met Gen Laurie".
The Iraq Inquiry, which is examining the decision to go to war and lessons learnt in Iraq policy between 2001 and 2009, also disclosed on Thursday that its final report will not be published until the autumn at the earliest.
In February its chairman refused to set a deadline but publication had been expected in the summer.
On Thursday, Sir John Chilcot said the committee now hoped "to present our report to the prime minister later this year but not before Parliament's summer recess".
"Writing a report covering so wide and complex a time period necessarily takes time.
"Whilst writing the report, we are also simultaneously seeking the declassification of much relevant material so the public will understand why and how the inquiry has reached its conclusions.
"If the Iraq Inquiry chooses to make criticisms, as is the case with all public inquiries, this would necessarily involve further processes to give those criticised the opportunity to respond.
"We cannot predict now how long that would take."