Sir John Chilcot has delivered a devastating critique of Tony Blair’s decision to go to war in Iraq in 2003, with his long-awaited report concluding that Britain chose to join the US invasion before “peaceful options for disarmament” had been exhausted.

The head of the Iraq war inquiry said the UK’s decision to attack and occupy a sovereign state for the first time since the second world war was a decision of “utmost gravity”. He described Iraq’s president, Saddam Hussein, as “undoubtedly a brutal dictator” who had repressed his own people and attacked his neighbours.

But Chilcot – whom Gordon Brown asked seven years ago to head an inquiry into the conflict – was withering about Blair’s choice to join the US invasion. Chilcot said: “We have concluded that 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.”

At times, Blair’s notes read more like stream of consciousness than considered policy documents. The note continued: “He [Saddam] is a potential threat. He could be contained. But containment … is always risky.”

According to Chilcot, Blair shaped his diplomatic strategy around the need to get rid of Saddam which – he told Bush – was the “right thing to do”. Blair suggested that the simplest way to come up with a casus belli was to give an ultimatum to Iraq to disarm, preferably backed by UN authority.

Chilcot rejects Blair’s view that spurning the US-led military alliance against Iraq would have done major damage to London’s relations with Washington. “It’s questionable it would have broken the partnership,” he writes, noting that the two sides had taken different views on other major issues including the Suez crisis, the Vietnam war and the Falklands.

The report also demolishes Blair’s claim made when he gave evidence to the inquiry in 2010 that the difficulties encountered by British forces in post-invasion Iraq could not have been known in advance.

“We do not agree that hindsight is required,” Chilcot says. “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.”