The 66-year-old career civil servant has served five prime ministers: Heath, Wilson, Thatcher, Major and Blair.
Why was the inquiry set up?
The Foreign Secretary Jack Straw set up the inquiry on 4 February 2004 following widespread public concern about the reliability of pre-war intelligence which claimed that Iraq had weapons of mass destruction. No such weapons have been found.
The inquiry members are: Lord Butler; Sir John Chilcot, a former civil servant in Northern Ireland; Labour MP Ann Taylor; Conservative MP Michael Mates; and Field Marshal Lord Inge, formerly Chief of the Defence Staff.
The committee met in private but had access to witnesses and documents.
Lord Butler himself has been criticised. Why?
His critics say that as a former top civil servant he is too close to government and will not rock the boat. They point to his too ready acceptance in 1994 of former Tory MP Jonathan Aitken's word about his dealings with a Saudi businessman. In fact Mr Aitken had lied and was later sent to prison for perjury.
His supporters say that his knowledge of government will help him make accurate assessments.
http://news.bbc.co.uk/1/hi/uk_politics/3878025.stm