Carne Ross told MPs the intelligence presented to the public about weapons of mass destruction was "manipulated".
He also added that "the proper legal advice from the Foreign office on the legality of the war was ignored".
Mr Blair has always defended the war's legality and the Butler inquiry said there was no evidence of "deliberate distortion" of intelligence on WMD....
His Butler testimony concluded that the invasion had been unlawful, he told the MPs in a separate, written submission. It also accused the government of misleading the public over the threat posed by Saddam, and of failing to consider alternatives to military action.
Speaking in public for the first time since he left the diplomatic service two years ago, Mr Ross also confirmed suspicions that the Prime Minister made up his mind months before the Iraq invasion in March 2003 that the war was going to happen and British troops would take part. Mr Ross said when he was serving in the embassy in Afghanistan, as early as April 2002, British officials there knew troops were being held back in readiness for the Iraq invasion.
He claimed that when official documents from the Foreign Office are made public, they will prove that the view of British officials, repeatedly conveyed to the Americans, was that overthrowing Saddam Hussein would cause chaos.
He told MPs: "I took part in the bilateral discussion between the State Department and the Foreign Office for four years. One of the items repeatedly on the agenda was regime change. Whenever that item came up, the leader of our delegation would say, with emphasis: 'We do not believe regime change is a good idea in Iraq. The reason we do not believe that is because we believe Iraq will break up and there will be chaos if you do that'. That view will have been recorded in the telegrams that have remained secret, and will do for years. That was emphatically the unified view of the Foreign Office.
"That view changed in mid-2002. There was no basis for changing the view from what was going on inside Iraq. What changed was our view of what the future policy would be."
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