Former Prime Minister Tony Blair should be tried for war crimes over the 2003 Iraq war if evidence shows he broke international law, Labour MP Jeremy Corbyn, a leading candidate to head the party Blair once led, has said.

In 2003, when Blair was Prime Minister, Britain joined a US-led invasion of Iraq despite widespread public opposition. At the time, Corbyn organised mass protests against the war.

"It was an illegal war. I am confident about that," Corbyn told BBC's Newsnight.

Asked if he thought Blair should face trial, Corbyn said: "If he has committed a war crime, yes. Everybody who has committed a war crime should be."

Blair has previously said he had no regrets over the invasion, saying former Iraqi dictator Saddam Hussein was a threat to the world who had to be disarmed or removed.

Corbyn's comments on Blair, Labour's most successful election winner, come during a fraught contest to find a new leader for the party which has become a tug-of-war between Corbyn, who wants to return Labour to its socialist roots, and others who want to fight for the centre ground Blair once dominated.

Corbyn said a long-running inquiry into the Iraq invasion needed to tackle unanswered questions about how Blair decided to get involved in the war.

Lengthy delays to the publication of the report have frustrated MPs, including Prime Minister David Cameron who on Tuesday urged its independent head, John Chilcot, to set out a timetable for the publication of his findings.