By Nick Assinder
Political correspondent, BBC News website
Revelations about Mr Blunkett's private life could threaten his career
David Blunkett's private life has, until now, remained pretty much off limits in Westminster.
The latest allegations, combined with his swift decision to launch an inquiry into them have now ensured, however, that it will dominate news in Westminster for some time to come.
The Home Secretary's supporters, and they currently include the prime minister, are confident the inquiry will clear him of any wrongdoing.
And Mr Blunkett has seen the danger of this issue providing a distraction for the government and has moved to bring it to a head as soon as possible.
Until now, he has attempted to keep his personal troubles private and the opposition parties had decided that, in this case, his personal life did not impinge on his public office and, as a result, should remain just that.
But the latest allegations changed all that and the home secretary's future is now being called into question.
So long as the controversy only surrounded his affair with a married woman there was little chance it would affect his public life.
That is not always the case, of course. Just ask former Tory frontbencher Boris Johnson.
But Mr Blunkett adamantly refused to get drawn into the media speculation in any way, and his privacy was widely respected by his political rivals.
However, claims that he abused his position as home secretary take these stories into an entirely different area and one of absolutely legitimate concern.
There are some small echoes of the incident that led to Peter Mandelson's second resignation from the government.
That too was over whether he had used his position to intervene in an application, this time over passports for the wealthy Hinduja brothers.
He was forced to resign almost immediately although he always maintained his innocence.
It was the way he handled the accusations that made his position untenable.
Prime Minister's confidence
Subsequent inquiries cleared him of any wrongdoing but the damage was already done and the affair hung over him for months.
And that is now one of the biggest problems for David Blunkett.
He retains the prime minister's confidence and appears to have handled the speculation in an open and straightforward manner.
Will the prime minister continue to stand by Mr Blunkett?
He also fiercely denies that he helped with the visa application for his ex-lover's former nanny - the most serious allegation.
Some have asked why he did not simply refuse to get involved in any way.
But whatever the outcome of the inquiry, he now knows this affair is not simply going to go away.
If, at the end of it, any of the allegations are upheld then his political career would almost certainly be over.
In any case, the distraction of a long-running investigation into the home secretary is the last thing the prime minister wants in the months leading up to the general election so he will be hoping the investigation will be swift.
The prime minister has a record of standing by his ministers in situations like this.
The details of the controversies surrounding Stephen Byers and Beverley Hughes, for example, may not be comparable to Mr Blunkett's position, but they also saw Mr Blair sticking by his ministers until the last moment.
In the end, when these affairs become damaging to the government itself rather than just the individual involved, or if they in any way impact on the prime minister's judgement, then all bets are off.