The Iraq War was an “act of military aggression, launched on a false pretext” and the “colonial style occupation” that followed led to the rise of ISIS, Jeremy Corbyn has said in response to the publication of the Chilcot Report.

Speaking in the House of Commons, the Labour leader said that the war had “long been regarded as illegal by the overwhelming weight of international legal opinion” but refrained from calling for legal consequences for Tony Blair.

Sir John Chilcot’s report did not make a judgement on whether or not the war was legal but did conclude that the legal basis for Britain’s military action was “far from satisfactory”.

Mr Corbyn, who passionately campaigned against the war, said that his response was only “provisional”, as he had only seen the report at 8am on Wednesday morning. The Labour leader will give a speech at 5pm on Wednesday, at which he may issue a formal apology of behalf of the Labour Party.

Paying tribute to the families of military personnel who lost their lives in the conflict, he said that the war had been “a catastrophe” that had led to “a fundamental breakdown in trust in government and our institutions”.

“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.

“It led to the deaths of hundreds of thousands of people and the displacement of millions of refugees. It devastated Iraq’s infrastructure and society. The occupation fostered a lethal sectarianism that turned into a civil war. Instead of protecting security at home or abroad the war fuelled and spread terrorism across the region.

“Sunday’s suicide bomb attack in Baghdad which killed over 250 people, the deadliest so far, was carried out by a group whose origins lie in the aftermath of the invasion.

“The invasion and occupation of Iraq has been, by any measure, a catastrophe.”

He added that Iraq’s current situation could be traced back directly to the “colonial style occupation” imposed by the US and the UK.

Mr Corbyn’s response to the report was watched closely by Labour MPs, who last week overwhelmingly backed a vote of no confidence in his leadership. At one stage he was heckled by Labour MP for Dudley North Ian Austin, who shouted “shut up” and “you’re a disgrace”.

A spokesman for Mr Corbyn refused to be drawn on the Commons protest, saying only that “Jeremy Corbyn was representing the party’s view as the democratically elected leader.”