During 37 years in public administration, Butler rose to become perhaps the ultimate establishment insider, eventually serving as head of the civil service under three different prime ministers.

The last of these was current premier Tony Blair, whose style of government was heavily criticised when Butler reported that the intelligence on Iraq's weapons of mass destruction was largely unreliable.

Blair, Butler found, was not personally to blame for the intelligence failings.

When he was appointed to lead the inquiry in February, Butler, now 66, was dismissed by some critics as being far too much of an unctuous mandarin to criticise ministers with any independence.

His background undoubtedly offers evidence for such a view. Educated at Harrow School, a favourite of the rich and well-connected, and then Oxford University, Butler joined the Treasury as a junior civil servant in 1961.

Rising steadily through the ranks, he became Cabinet Secretary - head of the entire domestic-based civil service - in 1988, a post he kept until retirement a decade later.

Publicly proved wrong

Butler's judgement was called into question during the 1990s when he investigated a pair of Conservative lawmakers accused of corrupt practices, Neil Hamilton and Jonathan Aitken.

Butler found little wrong in their conduct - only to be publicly proved very wrong when Hamilton resigned from the government for accepting money to ask parliamentary questions while Aitken was jailed for perjury.

Butler also found himself quite literally in the firing line on a couple of occasions.

In 1984 he narrowly escaped being killed when a bomb planted by the Irish Republican Army (IRA) destroyed part of a hotel in the coastal resort of Brighton where then-prime minister Margaret Thatcher and her ministers were staying.

Seven years later Butler flung himself to the floor alongside Thatcher's successor, John Major, as the IRA fired mortar rounds landed in the garden of Downing Street.

The incident is occasionally remembered as the only time contemporaries saw Butler behave with anything other than complete dignity.

Surprise choice

After his retirement, the grey-haired Butler, married for 42 years and with three children, was made a peer and became head of one of Oxford University's colleges.

Despite his pedigree, Butler was a surprise choice who many expected to side with the government, said Paul Kelly, a lecturer in politics at the London School of Economics.

"He's definitely an establishment figure but he's a very sensible and cautious, small "c" conservative figure, who will be aware of the longer-term reputation of government," Kelly told AFP when Butler was appointed.