Jeremy Corbyn, the Labour leader, has already said he will push for Blair to be prosecuted if he is found to have broken any laws.
Corbyn said last August: "We went into a war that was catastrophic, that was illegal, that cost us a lot of money, that lost a lot of lives, and the consequences are still played out with migrant deaths in the Mediterranean, refugees all over the region."
Asked if Blair should be charged with war crimes, Corbyn went on: "If he's committed a war crime, yes. Everyone who's committed a war crime should be.
"I think it was an illegal war, I'm confident about that. Indeed, [former UN Secretary-General] Kofi Annan confirmed it was an illegal war, and therefore [Blair] has to explain that. Is he going to be tried for it, I don't know. Could he be tried for it? Possibly."
Corbyn's aides signalled last week that he stood by these comments.
Reg Keys, whose son Lance Corporal Thomas Keys died in Iraq in 2003, said: "I do understand that [inquiry chairman] Sir John [Chilcot] cannot apportion blame but I hope he may suggest blame."