Jeremy Corbyn has apologised for the "disastrous decision" to invade Iraq in 2003, on behalf of the Labour party.
The Labour leader called the Iraq War the "most serious foreign policy calamity of the last 60 years", and said he owed an apology to the people of Iraq and the soldiers who had died or been injured in the conflict.
Speaking after a meeting with families of some of the British soldiers who had been killed, Mr Corbyn said the war was a "stain on our party and our country".
He said the decision to enter into Iraq was "long regarded as illegal" and had been launched on a "false pretext", after the long-awaited Chilcot report found that the invasion was not the "last resort" presented to MPs and the British public.
The inquiry revealed that Tony Blair had pledged to be with George W Bush "whatever" in a letter in 2002, and Mr Corbyn called for "stronger oversight" on future decisions to go to war.
A prominent figure in the opposition movement to the invasion, Mr Corbyn repeated his suggestions - first made in Parliament earlier this afternoon - that MPs had been lied to by Mr Blair's government.
Speaking in Church House in Westminster, he said MPs had been "misled" by a "small number of figures" who were committed to the conflict.
He did not directly name the former prime minister, but said: "All those who took the decisions laid bare in the Chilcot report must face up to the consequences, whatever they may be."
Giving his reaction to the report, Mr Blair said the decision to launch military action was "the hardest, most momentous and agonising decision" he made during his time in Downing Street.
He insisted he stood by the UK's involvement, telling journalists: "I believe that I made the right decision and that the world is better and safer as a result of it."
The Iraq War continues to deeply divide Labour, with one MP tweeting "not in my name" after the leader's formal apology.
Mr Corbyn, who formed Stop the War coalition, was heckled by some of his own MPs during his Commons response to the report.
He had previously pledged to issue the apology if elected as Labour leader.
Sir John Chilcot's report, released seven years after it was set up, concluded that the legal basis for UK military action in Iraq was "far from satisfactory".
The inquiry also said the threat posed by Iraq's weapons of mass destruction was "presented with a certainty that was not justified" by Mr Blair's government.
Sir John's highly critical report said the invasion was based on "flawed intelligence" which had not been challenged but "should have been", while preparation for the period after Saddam Hussein's fall was "wholly inadequate".
