Lord Turnbull, the former head of the civil service who has accused Chancellor Gordon Brown of "Stalinist ruthlessness", is known at Westminster for his formal, "quietly spoken" approach.
Lord Turnbull worked at the Treasury for more than 25 years
He has also been outspoken in his criticism of former civil service colleagues - notably ex-ambassador Sir Christopher Meyer - who broke ministerial confidences in their memoirs.
As he put it in December 2005 to the Commons public accounts committee Sir Christopher should be asked about "the effect of patronising and derogatory comments in relation to elected politicians whom an ambassador has been paid, and paid handsomely to serve".
"You may say it is all kind of airy-fairy old 'good chap' stuff", but, goes on, breaking confidences leads to damage being done."
That view appears to be the one held at Downing Street where Tony Blair's spokesman said that good government is damaged "when a civil servant becomes the story".
All in all, given his previous views and high-flying civil service career, Lord Turnbull's comments to the Financial Times, that Mr Brown has a "very cynical view of mankind and his colleagues", come as something of a shock.
Yet it is not the first time he has spoken out about his time in Whitehall.
In a BBC documentary broadcast earlier this month he was critical of the Labour leadership's attitude to Cabinet government and the civil service.
Lord Turnbull said: "Was too much policy developed at the centre? Yes, I think it was."
He added: "It's often a short-sighted view because if you are constantly taking something over you will not develop the capability and also people won't develop the sense of pride. They will actually feel belittled."
It is the outspoken tone of Lord Turnbull's latest comments on Mr Brown - the favourite to succeed Tony Blair as prime minister - which has most surprised political commentators.
Former Conservative minister Michael Portillo, who worked with him at the Treasury in the 1990s, remembers him as a "quietly spoken man. I hardly ever heard him speak above a whisper."
Lord Turnbull, born Andrew Turnbull in 1945, is an economist who spent more than 25 years at the Treasury.
He first joined in 1970, three years after graduating from Cambridge University and having worked for two years for the Zambian government.
While at the Treasury, he served as economic private secretary to Prime Minister Margaret Thatcher, known herself as a demanding and assertive boss.
In 1994 he moved to become permanent secretary to the Department of the Environment.
Working with Brown
Four years later he returned to become permanent secretary at the Treasury, now with Mr Brown as his boss.
Lord Turnbull's opinion of the chancellor seems to be a mixture of awe and disapproval.
He told the FT that Mr Brown had increased the power of the Treasury, but this had come "at the expense of any government cohesion and any assessment of strategy".
Lord Turnbull added: "You can choose whether you are impressed or depressed by that, but you cannot help admire the sheer Stalinist ruthlessness of it all."
The Treasury under Mr Brown viewed other departments with "more or less complete contempt", he added.
But Lord Turnbull praises some policies, such as granting independence to the Bank of England and setting up the three-year spending round.
His time working with Mr Brown seemed to do Lord Turnbull's career no harm either as, in 2002, he rose to become Cabinet Secretary and Head of the Home Civil Service.
And, since leaving that post in 2005 - becoming Baron Turnbull of Enfield - he has taken on a number of directorships at companies like Prudential, British Land Company, and Frontier Economics.
He is also an adviser to consultancy firm Booz Allen Hamilton.
Lord Turnbull has been married to wife Diane since 1967 and has two sons. He lists his hobbies as golf, opera and sailing.