“At no point did anybody from the prime minister down say to anybody within the intelligence services, ‘You have got to tailor it to fit this judgment or that judgment’ ” Mr. Campbell said. “The whole way through, it could not have been made clearer to anybody that nothing would override the intelligence judgments and that John Scarlett was the person, if you like, who had the single pen.”

Mr. Campbell’s testimony set the stage for Mr. Blair’s appearance before the inquiry later this month or early in February. The former prime minister has struck a similarly assertive tone, telling the BBC in an interview last month that he would “still have thought it right” to remove Saddam Hussein by force even if he had known that the Iraqi dictator had not possessed nuclear, biological or chemical weapons. He said Mr. Hussein and his government were a “threat to the region” even without unconventional weapons.

The inquiry, led by Sir John Chilcot, a retired official with close career links to Britain’s intelligence services, was established by Gordon Brown, Mr. Blair’s successor as prime minister, who promised to establish a wide-ranging investigation of Britain’s role in Iraq when he took the job in 2007. Two previous inquiries established under Mr. Blair were more narrowly focused, and reached conclusions that many war opponents felt were too sparing of Mr. Blair and other powerful officials, including Mr. Campbell, part of the inner circle that drove the decision to go to war.

Mr. Campbell is a former tabloid newspaper reporter who became powerful through his close personal relationship with Mr. Blair. Already a highly contentious figure, he resigned amid the controversy that erupted in the summer of 2003 after a leading British weapons specialist, David Kelly, whom Mr. Campbell and others accused of making misleading statements to a BBC journalist about the government’s statements on Iraq’s weapons, was found dead with his wrist slashed on a hill near his Oxfordshire home.

Mr. Campbell’s testimony was filled with instances of the uncompromising approach that caused even senior cabinet ministers to treat him with care. He was unrelenting in his backing for Mr. Blair and for the approach he took to removing Mr. Hussein. “You seem to be wanting me to say that Tony Blair signed up to say, ‘Regardless of the facts, regardless of W.M.D., we are just going to get rid of the guy,” he said. “It was not like that.”