Jeremy Corbyn has seized on the Chilcot Report as backing his assertion that the Iraq War was an act of military aggression based on a deliberate misrepresentation of the facts.
The UK Labour leader, who voted against the 2003 invasion, again insisted the conflict was illegal as he set out his response to John Chilcot's inquiry.
Mr Corbyn has been an anti-war activist for decades and previously chaired the Stop the War coalition, which was founded after former US president George W Bush set out his intentions to wage a "war on terror" in 2001.
Corbyn told the Commons: "The war was not in any way, as Sir John Chilcot says, a last resort.
"Frankly, it was an act of military aggression launched on a false pretext, as the inquiry accepts, and has long been regarded as illegal by the overwhelming weight of international legal opinion."
Meanwhile, MPs who voted to invade Iraq should take their "fair share" of the responsibility, the UK prime minister has said in the wake of the Chilcot report.
Making a statement in the House of Commons following the publication of the long-awaited report, David Cameron said there were lessons that needed to be learned from the Iraq war.
Mr Cameron said: "The decision to go to war came to decision in this House. Members on all sides who voted for military action will have to take our fair share of the responsibility.
"We cannot turn the clock back but we can ensure that lessons are learned and acted on."