The legality or illegality of the Iraq war was never a question Sir John Chilcot was asked to deal with in his long-awaited inquiry, Robin Butler has said.

Two days before the unveiling of the Chilcot report, a 2m-word document six years in the making, Lord Butler said on Monday: “What [Chilcot] was asked to deal with was what happened, not only in the lead-up to the war but during the war and after the war, and what lessons can be learned from it. The legal issue wasn’t actually put to him. His review team wasn’t equipped properly to deal with that issue.”

Speaking on the BBC Radio 4 Today programme, Butler, who chaired the 2004 Review of Intelligence on Weapons of Mass Destruction – or the Butler Review, as it became known – reiterated that former PM Tony Blair wanted to assist the US because he thought Saddam Hussein was a dangerous person to the world and the Middle East, and the world would be better off without him. “My criticism of him was about the way he reported the intelligence, he exaggerated the reliability of the intelligence … he was trying to persuade the United Nations and the world that there was a proper legal basis for taking military action against Iraq,” he said.

“In fact, the joint intelligence committee said to him the intelligence was sporadic and patchy. He said to the House of Commons that it was extensive, detailed and authoritative. That was where the inconsistency lay. I don’t call that a lie, he may himself have thought it was extensive, detailed and authoritative, but it wasn’t.”

Butler said he did not think Blair should have resigned when it was revealed that Saddam Hussein did not possess weapons of mass destruction, “because he really did believe what he was doing was right”.

He said that while there would not be a legal or illegal pronouncement by Chilcot, the point of conducting the inquiry was partly because there was a lot of pressure from the bereaved to see the people who had taken the decision brought to account, and partly to cover all the lessons to be learned politically and militarily.

“It is a vast remit. I think the problem for governments is that there’s always pressure for everything to be covered, governments find it difficult to say no, we’re going to narrow it down, because people then say well you’re covering something up or we’re not satisfied, so there always is that pressure. But I’m not surprised it took so long, because first of all there was the vastness of the remit, then there was the negotiations with the United States about what we could say about the president’s exchanges with Tony Blair, then there was the question of giving people who would be criticised the chance to make their representations, and finally clearing with the British government what could be published.”

He added that the main problem has been the inadequacy of the plan after the war, which was “largely down to the Americans, not to us”.

The likelihood of Blair being tried for war crimes in the wake of Wednesday’s report remains remote. In an official statement to the Telegraph, the international criminal court (ICC) indicated that the decision to go to war remained outside its remit, which means individual soldiers could be prosecuted for war crimes but not Blair.

Senior figures from Labour and the Scottish National party are considering calls for legal action against Blair if he faces severe criticisms from the inquiry. A number of MPs led by Alex Salmond, the former Scottish first minister, are expected to use an ancient law to try to impeach Blair, last used in 1806 when the Tory minister Lord Melville was charged for misappropriating official funds.

Salmond said there “has to be a judicial or political reckoning” for Blair’s role in the Iraq conflict while the shadow chancellor, John McDonnell, said the “processes” of how Britain ended up at war must be examined “so we never ever get into this tragic, tragic mess again with such loss of life”.

Blair refused to comment on Chilcot before it is made public, telling Sky News: “Wednesday is the time the report is published. I have said many times over these past years I will wait for the report and then I will make my views known and express myself fully and properly.”

Relatives of some of the 179 Britons killed in the Iraq conflict fear the report will not give them the answers they desperately want.

Gary Nicholson, 42, was one of 10 servicemen who died when their Hercules C-130 aircraft was shot down in 2005. His mother Julia told the Press Association: “It will be a whitewash. I’m absolutely disgusted. I’m not going because it will be a whitewash. Tony Blair has got blood on his hands. He will have covered his back and [George] Bush’s back.”

Janice Procter, whose son Michael Trench was one of the youngest British soldiers to die in Iraq when he was killed aged 18 in 2007, said: “It’s been horrendous, I’m very apprehensive about this. This man [Blair] has put 179 kids to the slaughter – there’s no justice. It [the report] is not going to give me any closure or comfort. I’m not going down on the day, I’m not going to waste two hours of my life reading it.”

Roger Bacon, whose son Matthew, 34, was killed in 2005, said: “It’s been hanging over our heads – a great rock sitting over our heads and it wears you down, no doubt about it, and has worn us down for a long time.”