In a written statement submitted to the inquiry, Blair said he had given then US President George Bush a "strong commitment" in January 2003 that Britain would do "what it took" to disarm Saddam.
He conceded that he had received advice from then attorney general Peter Goldsmith on 14 January and 30 January 2003, suggesting that a further UN resolution was required for military action to be legal, but said this was "provisional".
He noted that Goldsmith, the government's top legal adviser, changed his mind about the legality of invading Iraq. If he had not, "then the UK could not and would not have participated in the decision to remove Saddam", Blair said in the statement.
Goldsmith has criticised Blair for publicly suggesting Britain could invade without further UN backing, despite his advice.
“The issue was very simple,” Blair told the inquiry. “He either had a change of heart or regime change was on the agenda. We saw no link between Saddam and Iraq but post-September 11, WMD took on a different significance”’
Blair also defended his decision to refuse to release secret memos he wrote to Bush.
"The notes to President Bush were very private,” he said. “They were written when I wished to get a change or adjustment to policy. They had to be confidential. The personal relationship was a vital part of the country's strategic relationship."
The inquiry asked Blair why Iraq was not discussed in cabinet meetings between April and September 2002. He said he was surprised to hear that, but that cabinet members would have to have stopped reading the newspapers in order to have failed to know what Blair’s stance was.
“Suddenly we were going to go into an alliance with a right-wing, Republican president,” said Blair. “That was the most difficult thing in a way, politically.”