The report was critical of intelligence agencies, which were working with an "ingrained belief" that Saddam retained chemical and biological warfare capabilities which he was hiding from UN inspectors and that he was determined to acquire nuclear weapons.

The Joint Intelligence Committee (JIC) should have made clear to Blair that its suspicions about WMD had not been established "beyond doubt" prior to his publication in September 2002 of a dossier setting out the supposed threat from Saddam, Sir John found.

As late as March 17 2003 - three days before the invasion began - JIC chairman Sir John Scarlett continued to advise the PM that Iraq possessed chemical and biological weapons and the means to deploy them.

But Sir John said: "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 findings of the 2004 Iraq Survey Group into Iraq's WMD capabilities were "significant" but did not support pre-invasion statements by Blair and then foreign secretary Jack Straw, which warned of vast stocks of weapons and an urgent and growing threat.

Blair's response that Saddam retained "the intent and the capability" to develop and use WMD did not match the justification for military action given before the conflict, said Sir John.

The report found that Blair urged Bush in the immediate aftermath of the September 11 terror attacks in 2001 not to take "hasty" action against Iraq.

But it said by the time of the Crawford meeting, there had been a "profound change" in his thinking, with the UK Government stating openly that Iraq was a threat that had to be dealt with, and the JIC privately concluding that Saddam could not be removed without an invasion.

The Government's insistence that Iraq must disarm or be disarmed implied the readiness to use force if Baghdad did not comply, and contingency planning for an invasion had indeed begun, the report found.

In his July 28 letter, Blair told Bush that a coalition for military action would be dependent on the authority of the UN, and a resolution was passed by the Security Council in November 2002 giving Iraq a final opportunity to disarm to avert war.

But Sir John found that by December, Bush had decided military action would take place anyway in early 2003 - a timetable which Blair accepted at the end of January.

Although Blair and Straw subsequently blamed France for the failure to secure a resolution backing war and claimed to be acting to uphold the authority of the Security Council, the report found that "in the absence of a majority in support of military action, we consider that the UK was in fact undermining the Security Council's authority".

The circumstances under which Lord Goldsmith provided a legal justification for UK involvement were "far from satisfactory", the report found.

Having initially advised Blair in January 2003 that a second UN resolution was necessary, he later produced written advice that there was a "reasonable case" that the November resolution alone was sufficient.

On March 14, he asked Blair to confirm that Iraq had breached the terms of the first resolution, which required Baghdad to provide access to UN weapons inspectors. The precise basis on which Blair gave that crucial confirmation "is not clear", said the report, which added that the issue should have been discussed by Cabinet.

The report dismissed Blair's argument that the emergence following Saddam's fall of a violent insurgency against occupying allied forces could not have been foreseen.

"We do not agree that hindsight is required," stated Sir John. "The risks of internal strife in Iraq, active Iranian pursuit of its interests, regional instability and al Qaida activity in Iraq were each explicitly identified before the invasion."

Blair "did not establish clear ministerial oversight of UK planning and preparation, he did not ensure that there was a flexible, realistic and fully-resourced plan that integrated UK military and civilian contributions and addressed the known risks".

Sir John acknowledged the "deep anguish" of the families of those killed and injured in Iraq, many of whom were present in London to read his report for themselves.

And he said the people of Iraq had "suffered greatly" from the failure to deliver on the vision of a peaceful, secure and democratic Iraq set out by the US and its allies in the eve-of-war summit in the Azores. At least 150,000 and probably many more had died, and one million had been forced from their homes.

The UK took responsibility for south-eastern Iraq without a formal ministerial decision and "without ensuring that it had the necessary military and civilian capabilities to discharge its obligations, including, crucially, to provide security", found Sir John.

"The scale of the UK effort in post-conflict Iraq never matched the scale of the challenge. Whitehall departments and their ministers failed to put collective weight behind the task. In practice, the UK's most consistent strategic objective in relation to Iraq was to reduce the level of its deployed forces."

The MoD was slow in responding to the threat from improvised explosive devices (IEDs) used by insurgents to inflict multiple casualties on occupying forces.

And from 2006, when UK troops were deployed in the Helmand province of Afghanistan, the armed forces were conducting two campaigns without "sufficient resources" to do so.

Decisions on resources for Iraq were affected by the demands of the operation in Afghanistan, with a "material impact" on the availability of essential kit like helicopters and surveillance equipment.

The inquiry panel agreed unanimously that, while military action in Iraq "might have been necessary at some point", at the time the invasion was launched, there was "no imminent threat from Saddam Hussein", "the strategy of containment could have been adapted and continued for some time" and "the majority of the Security Council supported continuing UN inspections and monitoring".

Sir John urged the Government to learn lessons from the mistakes made over Iraq, including:

:: The importance of collective ministerial discussion with "frank and informed" debate;

:: The need to assess risks and set achievable and realistic strategy;

:: The vital role of ministerial leadership and co-ordination of action across government; and

:: The need to ensure that the military - and civilian officials - are properly equipped.

"Above all the lesson is that all aspects of any intervention need to be calculated, debated and challenged with the utmost rigour," said Sir John. "And when decisions have been made, they need to be implemented fully.

"Sadly, neither was the case in relation to the UK government's actions in Iraq."