“The decision to go to war in Iraq has been a stain on our party and our country,” the Labour leader said after apologising at a private meeting with families of some of the 179 British servicemen and women killed in Iraq, veterans of the military operation and Iraqis who lost family members.

Speaking at Church House, Westminster, Mr Corbyn went further than he did five hours earlier in the Commons, where he stopped short of making an apology. But on neither occasion did he raise the prospect of legal action against Mr Blair, as some families want.

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Pointing the finger of blame at Mr Blair, Mr Corbyn said Labour MPs who, unlike him, voted for the war “were misled by a small number of leading figures in the Government who were committed to joining the US invasion of Iraq come what may and were none too scrupulous about how they made their case for war”.

Mr Corbyn declared: “So I now apologise sincerely on behalf of my party for the disastrous decision to go to war in Iraq in March 2003. That apology is owed first of all to the people of Iraq. Hundreds of thousands of lives have been lost and the country is still living with the devastating consequences of the war and the forces it unleashed. They have paid the greatest price for the most serious foreign policy calamity of the last 60 years.”

He went on: “The apology is also owed to the families of those soldiers who died in Iraq or who have returned home injured or incapacitated. They did their duty but it was in a conflict they should never have been sent to.”

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Mr Corbyn said: “Finally, it is an apology to the millions of British citizens who feel our democracy was traduced and undermined by the way in which the decision to go to war was taken on the basic of secret ‘I will be with you, whatever’ understandings given to the US president that have now been publicly exposed.”