By Nick Assinder
Political Correspondent, BBC News website
The man charged with the task of deciding Tessa Jowell's future has already shown he will not shrink from the task.
Cabinet Secretary Sir Gus O'Donnell carried out a similar inquiry into David Blunkett's affairs last November and it was his advice to the prime minister that ensured the minister's second resignation.
Sir Gus will decide minister's future
He came into the top Whitehall job, which includes oversight of the ministerial code, only eight months ago and immediately declared his top priority was to restore public trust in politics and Whitehall.
He had to live up to that promise within a few weeks, over the Blunkett affair, and his swift and decisive action saw many claiming he was flexing his muscles in an unprecedented way.
It is similarly claimed that by starting an inquiry into Ms Jowell's affairs in response to a letter from Tory Theresa May, rather than a direct request from the prime minister, is another example of his growing power - although it is also claimed he would not have acted without the prime minister's say so.
Either way it is Sir Gus's advice that will decide Ms Jowell's immediate future.
Few people know the internal workings of Whitehall and government as comprehensively as he does.
He has huge experience in Whitehall working for ministers, and chancellors, from both sides of the political divide.
Major forged friendship with O'Donnell
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 Mr O'Donnell 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).
The cabinet secretary 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.
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.