Alastair Campbell was "somewhat of an unguided missile" during talks about intelligence before the Iraq war, a senior MI6 officer has said.
Tony Blair's former spokesman tended to have "rushes of blood to the head", previously secret evidence released by the Iraq inquiry suggests.
The officer, who cannot be named, said the pre-war atmosphere had been "febrile" and MI6 had made mistakes.
The UK invaded Iraq in March 2003. The inquiry is due to end later this year.
Chaired by Sir John Chilcot, it is examining events between 2001 and 2009, including the decision to go to war in Iraq, post-invasion planning, what lessons were learned and whether troops were properly equipped.
In evidence given during a closed hearing, the MI6 officer said Mr Campbell had had regular contacts with MI6, the Secret Intelligence Service (SIS), as part of the government's communications strategy in the run-up to the March 2003 invasion of Iraq.
The witness, referred to as "SIS2", told the inquiry: "We found Alastair Campbell, I think, an enthusiastic individual, but also somewhat of an unguided missile."
He added: "We also, I think, suffered from his propensity to have rushes of blood to the head and pass various stories and information to journalists without appropriate prior consultation.
"That's not to say that we didn't engage to the extent that we could, and I think that Alastair Campbell found us a useful organisation to work with, simply because SIS was actually an organisation that was very focused on delivery."
SIS2 admitted that the "very febrile atmosphere" before the war resulted in MI6 making mistakes, including validating some intelligence that later had to be withdrawn after it turned out the sources were not reliable.
He said it was a "fair criticism" to say the spy agency had been "probably too eager to please" 10 Downing Street.
The officer added: "The pressure to generate results, I fear, did lead to the cutting of corners. I think perhaps SIS was at that point guilty of flying a bit too close to the sun."
In testimony to the inquiry last year, former MI5 head Baroness Manningham-Buller said Mr Blair's government had used "fragmentary" intelligence, largely from MI6, to make the case for war.
In May, Mr Campbell hit back after a former intelligence official challenged his evidence to the Iraq inquiry.
Major General Michael Laurie disputed his claim that a September 2002 dossier was not meant to "make the case for war", saying that those producing it "saw it exactly as that and that was the direction we were given".
But Mr Campbell, who was Tony Blair's communications chief from 1997 to 2003, said those "directly involved" in producing the dossier had backed him.
He played a key role in creating the document, which contained the controversial claim that Iraqi leader Saddam Hussein's weapons of mass destruction could be deployed within 45 minutes of an order to use them.
The September 2002 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 had 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.