LONDON — Then-prime minister Tony Blair of Britain went to war alongside the United States in Iraq in 2003 on the basis of flawed intelligence that went unchallenged, a shaky legal rationale, inadequate preparation and exaggerated public statements, concluded an independent inquiry into the war in a report published yesterday.

The long-awaited report by the Iraq Inquiry Committee, led by Mr John Chilcot, takes up 12 volumes covering 2.6 million words and took seven years to complete, longer than Britain’s combat operations in Iraq. It concluded that Mr Blair and the British government underestimated the difficulties and consequences of the war and overestimated the influence he would have over President George W Bush.

The result amounts to a broad indictment of Britain’s involvement in the Iraq War — that overthrew Saddam Hussein — and its aftermath, and it portrays Mr Blair as trying without success to restrain Mr Bush, push him to obtain full United Nations Security Council authorisation and warn about the difficulties of the war, then deciding to go to war alongside Washington nonetheless.

Judging that Britain should stand by the US, Mr Blair told Mr Bush in a private note in July 2002, that “I will be with you, whatever”. Mr Blair knew by January 2003 that Washington had decided to go to war to overthrow Hussein and accepted the US timetable for the military action by mid-March, pushing only for a second Security Council resolution that never came, “undermining the Security Council’s authority”, concluded the report .

The report is likely to underline in Britain the sense that Mr Blair was “Washington’s poodle”, a phrase used by Mr Blair’s critics. The report says the lessons from the British government’s conduct are that “all aspects” of military intervention “need to be calculated, debated and challenged with the utmost rigor”, and decisions, once made, “need to be implemented fully”.

Mr Chilcot, speaking for the inquiry as a whole, concluded that “sadly, neither was the case in relation to the UK government’s actions in Iraq”. And he emphasised that Britain’s relationship with the US was a strong one. It is able “to bear the weight of honest disagreement”, he said. “It does not require unconditional support where our interests or judgments differ.”

He added: “It is now clear that policy on Iraq was made on the basis of flawed intelligence and assessments. They were not challenged, and they should have been.”

The report said: “The UK chose to join the invasion of Iraq before the peaceful options for disarmament had been exhausted. Military action at that time was not a last resort.”

In the end, the British government “failed to achieve its stated objectives”, the inquiry concluded, and said that “Mr Blair overestimated his ability to influence US decisions on Iraq”.

Mr Blair was advised by his diplomats and ministers of “the inadequacy of US plans” and their concern “about the inability to exert significant influence on US planning”. But he chose to override their objections.

The inquiry did not make any judgment on legal culpability. Outside the convention centre where Mr Chilcot spoke, near Parliament, demonstrators chanted and held up a sign reading: “Blair Must Face War Crimes Trial”.

In a brief statement, Mr Blair said: “The report should lay to rest allegations of bad faith, lies or deceit. Whether people agree or disagree with my decision to take military action against Saddam Hussein; I took it in good faith and in what I believed to be the best interests of the country.” THE NEW YORK TIMES