The 2003 invasion of Iraq was not necessary and failed to meet its stated objectives, the long-awaited Chilcot report has concluded. In a fiercely critical assessment, the 2.6 million word inquiry rebuked Tony Blair for all but committing the UK to military action in July 2002 while failing to agree or discuss the proposal with colleagues.

A note sent by the former prime minister to President Bush on 28 July stated that “I will be with you, whatever” and identified the likely failure of UN diplomacy as the simplest “casus belli”. The report concluded that Blair’s commitment, “which had not been discussed or agreed with colleagues”, set the UK on a path towards military action in a way that would make it “very difficult” for Britain “subsequently to withdraw its support for the US”.

Sir John Chilcot’s inquiry, published seven years after it was commissioned by Gordon Brown’s government, also criticised the failure to provide the cabinet with “written advice” explaining the belief that the invasion was lawful. The report said that former attorney general Peter Goldsmith’s full assessment should have been given to “ministers and senior officials”, rather than merely its conclusions. Blair approved UK military action after ruling that Iraq had committed “further material breaches” of UN Resolution 1441 and that a second resolution was not required. As late as mid-January 2003, two months before the invasion began, Goldsmith had stated that the latter was a necessity.

In a statement at the QEII Centre in Westminster, Chilcot emphasised that “the inquiry has not expressed a view on whether military action was legal” and that this “could, of course, only be resolved by a properly constituted and internationally recognised court.” Labour leader Jeremy Corbyn is set to apologise for the war on behalf of his party but is not expected to call for Blair’s prosecution.

The report did not accuse the former prime minister of intentionally misleading parliament over Iraq’s alleged possession of weapons of mass destruction. But it concluded that the government’s statements from late 2001 onwards “conveyed more certainty” than intelligence assessments allowed. It said: “The tendency to refer in public statements only to Iraq’s ‘weapons of mass destruction’ was likely to have created the impression that Iraq posed a greater threat than the detailed JIC Assessments would have supported.” The report lamented that this had left “a damaging legacy”, which “may make it more difficult to secure support for government policy, including military action”.

Chilcot said that the subsequent finding of the Iraq Survey Group that Saddam Hussein retained the “strategic intent” to reconstitute his WMD was “significant”. But he added that this “did not support pre-invasion statements by the UK government, which had focused on Iraq’s current capabilities”. The report noted that “the explanation for military action put forward by Mr Blair in October 2004 drew on the ISG’s findings, but was not the explanation given before the conflict.”

In military planning for the invasion from January 2003, Chilcot found that “the decision to deploy a large scale force” was “taken without collective ministerial consideration of the decision and its implications”. This led to “serious equipment shortfalls when conflict began”. One hundred and seventy nine UK troops and at least 150,000 Iraqi civilians were killed during the invasion and occupation.

In planning for a post-Ba’athist Iraq, the report concluded that Blair “did not press President Bush for definite assurances about US plans, did not consider or seek advice on whether the absence of a satisfactory plan called for reassessment of the terms of the UK’s engagement and did not make agreement on such a plan a condition of UK participation in military action”. Chilcot rejected Blair’s suggestion that the post-conflict difficulties could not have been known in advance. He said: “We do not agree that hindsight is required. The risks of internal strife in Iraq, active Iranian pursuit of its interests, regional instability, and al-Qaeda activity in Iraq, were each explicitly identified before the invasion.”

Read more: 5 times the Chilcot report turned the screws on Tony Blair

After achieving the removal of Saddam Hussein’s regime in less than a month, the report concluded that the UK and its allies subsequently failed to achieve their objectives. The government, it found, did not “plan or prepare for the major reconstruction programme required in Iraq”. Chilcot said that the UK’s acceptance of “particular responsibility” for four provinces in the south east was taken “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.”

The inquiry found that the Ministry of Defence “was slow in responding to the developing threat in Iraq from Improvised Explosive Devices” but that “it was not sufficiently clear which person or department within the MOD had responsibility for identifying and articulating capability gaps”. Chilcot added: “It should have been”.

The later deployment of troops to Afghanistan, the report found, had “a material impact” on the “availability of essential equipment in Iraq”, particularly helicopters and surveillance and intelligence equipment.

Chilcot denounced the “humiliating” outcome which led the UK to exchange “detainee releases for an end to the targeting of its forces”. He said: “It was humiliating that the UK reached a position in which an agreement with a militia group which had been actively targeting UK forces was considered the best option available.”

While conceding that military action in Iraq “might have been necessary at some point”, Chilcot said that the “unanimous view” of the report was that this was not the case in March 2003. He emphasised that “there was no imminent threat from Saddam Hussein”, that “the strategy of containment could have been adapted and continued for some time” and that “the majority of the Security Council supported continuing UN inspections and monitoring”.

The inquiry found that Blair’s decision to stand “shoulder to shoulder” with the US following the 11 September 2001 attacks was critical to UK participation in the invasion. But Chilcot contended that “the UK’s relationship with the US has proved strong enough over time to bear the weight of honest disagreement. It does not require unconditional support where our interests or judgements differ.”