British officials backed the purge of Saddam-era officials, the US administrator of post-invasion Iraq has told the Chilcot inquiry.
Senior British diplomats and Army officers have blamed Iraq's post-war descent into chaos on Paul Bremer's policy of de-Baathification.
But Mr Bremer told the inquiry UK officials agreed with it at the time.
Sir John Chilcot's inquiry met 15 leading figures on a five-day visit to the US last week.
Unlike in the UK, where most of the evidence sessions have been televised, the discussions, conducted between 17 and 21 May, were held behind closed doors.
They were described by the inquiry as private discussions and, as such, transcripts will not be published.
A spokesman for the inquiry said that they were intended "to allow the committee to receive a wider international perspective on the UK's involvement in Iraq over the period being examined by the inquiry".
Mr Bremer is the most high profile US figure to speak to the panel and the only one to release a statement, which has been published on the inquiry's website.
In it, he goes over much of the same ground he covered in his book on his year in charge of the Coalition Provisional Authority, saying the CPA's efforts were "chronically" under-resourced and coalition forces were "unable to provide adequate security for Iraqi citizens" after the March 2003 invasion.
But he also stresses that British officials backed the disbanding of the Iraqi army, saying "became evident in early April that the former army was no longer intact".
He said British officials were briefed on the plan in London by the CPA's senior adviser Walter Slocombe on 13 and 14 May 2003.
"His (Slocombe's) British interlocutors recognized that demobilisation was a fait accompli.
"None of them expressed the view that the Coalition should instead try to recall the Iraqi army.
"In fact, Slocombe reported that the British officials agreed with the need for vigorous de-Baathification, especially in the security sector."
In his evidence to the Chilcot Inquiry in December, Sir Jeremy Greenstock, the former British ambassador to the UN, said the US had been over-optimistic in its attitude towards rebuilding Iraq after the invasion.
He said Mr Bremer had refused to appoint him as his deputy, claiming that Britain had been cut out of the post-war decision-making process.
"He did not want to hand over to a non-American as acting administrator when he left the country to return to Washington," said Sir Jeremy.
A live television feed of Sir Jeremy's evidence was cut for more than a minute as he discussed post-war planning. Sir John said he interrupted the broadcast as it contained "sensitive information".
Others questioned by the Chilcot inquiry at a series of meetings in Washington and Boston included the former US ambassador to Iraq Ryan Crocker.
The panel also spoke to David Kay and Charles Duelfer who led the Iraq Survey Group, which conducted a fruitless search for weapons of mass destruction in the aftermath of the 2003 war.
The independent inquiry into the UK's involvement in Iraq was commissioned by former Prime Minister Gordon Brown in June 2009 and is expected to report later this year.
It has already taken evidence from witnesses including Tony Blair and Mr Brown himself, but public hearings were suspended during the recent general election.