By Nick Assinder
Political Correspondent, BBC News website
If Tony Blair had wanted to prove he was not politicising the civil service he could have done nothing better than appoint Sir Gus O'Donnell to run it.
Sir Gus is a civil servant to his very fingertips - in the best sense possible.
He has huge experience in Whitehall working for ministers, and chancellors, from both sides of the political divide.
His most public appointment was when Sir John Major took him to Downing Street as his press secretary after working with him as chancellor where the two had forged a friendship.
In the way Margaret Thatcher's press spokesman, Sir Bernard Ingham, was a near extension of his boss - to the point he often did not even need to hear her view in order to accurately reflect it - so Gus was to John Major.
Yet he was never viewed as an overtly political press secretary in the same way as Sir Bernard or, more obviously, Alastair Campbell.
The political journalists he dealt with on a daily basis universally respected his professionalism and the fact he would give a straight answer to a straight question.
And it was that which once or twice got him into warm, if not hot water and ultimately, it is believed, persuaded him this was not the path he wanted to take.
Britain also crashed out of the ERM at this time - not a happy period in Downing Street.
Sir Gus then returned to the civil service where his steady rise has taken him to the very top.
None of this is to suggest Sir Gus is a grey figure.
Indeed, few of us who were with him during one prime ministerial visit to an Eastern European country in the 1990s will forget the glee and enthusiasm he displayed when playing roulette. (He had a system which appeared to actually work - a debatable skill for a Treasury mandarin perhaps).
Sir Gus grew up in Vauxhall, across the river from the Houses of Parliament, before studying economics at Warwick and then Oxford. He briefly lectured at Glasgow University before joining the Treasury in 1979.
In his Treasury career, he was seconded to Washington in the early 1980s before working for Nigel Lawson, then Mr Major when they were chancellor.
Major forged friendship with O'Donnell
After leaving Downing Street in 1994, he became deputy director of the Treasury and then served as the UK's representative on the IMF and the World Bank while working at the UK Embassy in Washington.
When Labour came to power in 1997 he was persuaded to return to Whitehall, sharing power with Ed Balls, Gordon Brown's long-standing political advisor who became chief economic adviser in 1999.
In November 1997, he was made managing director, Macroeconomic Policy and International Finance, and head of the Government Economics Service, in charge of long-range forecasting with a team of more than 500 economists.
In 2002 he was appointed permanent secretary at the Treasury.