This has not been a bad week for David Blunkett. That might sound odd, given the uproar over his uncompromising comments about colleagues and civil servants and the revelation of a past affair with a much younger member of staff. But in a truly bad week the Home Secretary would have had to repair to his Derbyshire cottage to contemplate life on the backbenches. For his luck in escaping such a fate he has the Prime Minister to thank.

On their trip to Sheffield Tony Blair said of Mr Blunkett: “I’m proud to stand beside him and I’m proud to call him my friend” — a ringing endorsement. Mr Blair would also be right to feel let down by his Home Secretary and to reflect, once again, that while it is not difficult to find intelligent politicians, it is very difficult indeed to find intelligent politicians who are also in possession of sound judgment.

Mr Blunkett appears guilty of serial poor judgment and it has left him sorely damaged. Leaving aside the continuing inquiry into claims that he fast-tracked a visa for his former lover’s nanny, his messy affair with another woman, a civil servant 20 years his junior, whose partner also worked for him at the Department for Education, was inappropriate. Extracts from a book by Stephen Pollard, to whom he appears to have sung like a hyperactive canary, portray a man whose confidence borders on hubris. The barbs he aims at Cabinet colleagues — Gordon Brown is a “bully”; Jack Straw left the Home Office in a “giant mess” — and his condemnation of “incompetence” among civil servants were fascinating and not necessarilly inaccurate, but reflected wonderfully poor judgment — wonderful for newspapers.

There will be a loss in his standing among colleagues and a wariness among civil servants not in his close coterie. He may want to be Chancellor, but it is impossible to imagine Mr Blair awarding him such a prize when he has insulted so many who sit in Cabinet with him. Those left unscathed by his loose tongue will hardly be inclined to trust him. Every time he turns on his famous charm they must be wondering what he is really thinking and how in future he might lambast them in private, or when a clever biographer’s tape recorder is running.

But the person who must feel most upset is Mr Blair. Oliver Letwin’s obvious delight in airing the Pollard material on Question Time will be the start of a concerted attack on a Cabinet with many colourful rivalries between now and the general election. The story has broken Alastair Campbell’s “13-day rule” which states that an awkward story must be shut down before it has run for two weeks or it will begin to register strongly with voters. Every time a victim of Mr Blunkett’s scorn is interviewed, he or she will have the tricky task of attempting to frame a convincing response.

Mr Blair has told Mr Blunkett to mend fences and in the short term is backing him. Mr Blunkett’s long-term political prospects are less favourable and for just how long the embarrassed Prime Minister can actually remain so steadfast in his support should become clear in the next few days.