I have admired David Blunkett for years. As his biographer, I have studied the three decades of his political career with something close to awe. I have always regarded him as a shining example of achievement.

But reading his statement yesterday, one thing is clear. His behaviour stinks. No apology. No regret. No admission that he has done anything in the least beat wrong, or even the slightest error of judgement. Just an attempt to blame others for his own mistakes. He has, he said, asked his sons to sell the shares in DNA Bioscience to protect “family and friends from further intrusion”.

The intrusion, such as it is, is entirely down to one man, and one man’s behaviour. That man’s name is David Blunkett.

You know when it’s all over for a politician when they have to resort to technical defences. When the news first broke that David Blunkett had failed to consult the Independent Advisory Committee (the body set up to deal with former ministers’ subsequent employment) about his position on the board of DNA Bioscience, he said his understanding was that any obligation to tell the committee is entirely voluntary.

Come off it, Mr Blunkett. Since when was sticking to the letter but breaking the spirit of the rules OK?

Let’s not forget that he didn’t resign in December because he fancied a holiday. He resigned in disgrace. As such, it was surely more important than ever that his behaviour was above reproach.

Yet his argument now - that he didn’t tell the committee because he wasn’t legally obliged to do so - actually makes his behaviour seem worse. It makes it seem as if he didn’t tell them because, not having to, he thought he could somehow get away with it. “With hindsight it might have been better [to tell the committee]”, Mr Blunkett said when the news emerged. Actually, no. It would be crystal clear to almost anyone – anyone except Mr Blunkett, that is – at the time that he should have consulted the committee. The reason why David Blunkett has become a walking disaster area is because the appropriate behaviour appears no longer even to cross his mind.

It gets worse. His behaviour with regard to his directorship of DNA Bioscience has been merely stupid. On its own, it would add to the impression that he has lost his judgement, but would not be a resigning issue. But what takes it into a different order of magnitude is his decision to keep his shares in the family – until forced, last night, to sell them.

The family share ownership may have been legal. It may comply with the letter of the ministerial code of conduct. But it certainly does not comply with the spirit of the sensible, decent behaviour which we should be able to expect from our politicians – let alone from a minister who was brought back into the government a matter of weeks after having to resign in disgrace. At best it is shady behaviour.

This government came to office on the back of a wave of anger at ‘Tory sleaze’. But no serving Conservative minister behaved as sleazily as Mr Blunkett has done.

He was perfectly entitled to buy the shares when he bought them. He was out of office and had no idea if he would be back in government, let alone at what department. But as soon as he became Secretary of State for Work and Pensions, and so in charge of a department which could transform DNA Bioscience’s fortunes – and thus the worth of the shares – by awarding it contracts, it is surely obvious to everyone except Mr Blunkett that he ought to have made a clean break and sold the shares.

Not given them to someone else. Not gone through a technically correct procedure. Not stood back and hoped that their value – and his wealth - would go through the roof. But got rid of them. Completely.

That he now seems unable to comprehend that his whole arrangement with DNA Bioscience stinks says all that we need to know about the collapse of his once greatest asset – his judgement.

For almost thirty years, since he became chairman of Sheffield’s social services committee in 1976, his most striking feature has been his knack of knowing the right thing to do, whether it was with whom to ally politically, what to say, or how to behave. He had superb and instinctive judgement.

However bad his personal judgement may now be, his political judgement remains as acute as ever. Indeed, his row with Tony Blair over benefit cuts is classic Blunkett. A leaked memo from the Prime Minister ordered Mr Blunkett to go a lot further than he has apparently proposed – to which the Work and Pensions Secretary responded with an intemperate letter defending his position.

But the row has almost nothing to do with its supposed subject matter, benefit reform. It is, rather, about Mr Blunkett attempting to position himself for life after Mr Blair. Throughout his time in government, he has been seen – rightly – as the Prime Minister’s soulmate. But with Mr Blair’s political strength disappearing before our eyes, Mr Blunkett is keen to be seen as his own man so that he can continue with a big job once Gordon Brown takes over.

Holding out against extended cuts is also intended to bolster his position with the Labour backbenchers. The last minister who proposed real cuts, Harriet Harman, was savaged by her colleagues. Mr Blair was unable to protect her even in his pomp. With the Prime Minister now becoming less powerful by the day, he is still less able to protect his allies. So Mr Blunkett has decided he will not suffer the same fate, and will not propose serious cuts.

But all this now looks academic. His political antennae may be in full working order, but his personal judgement has deserted him since the beginning of his affair with Kimberly Quinn - when he started (some would say quite rightly) to put his private happiness above his political career.

That has led to all the crises which have subsequently engulfed him, from the original abuse of his position over the visa application for his son’s nanny, to the farce of his relationship, however platonic, with blonde estate agent Sally Anderson, to the latest mess over his involvement with DNA Bioscience. Where once he had a sure touch, and would have run a mile from such mistakes, today he seems to plunge headfirst into embarrassments.

David Blunkett has patented his own unique and ongoing form of crisis. His behaviour may not be technically wrong, but to outside, objective observers it just seems wrong. It leaves a bad taste in the mouth.

As this latest crisis engulfs him, the question has to be asked: how can a man who no longer seems to know how to behave remain as a Cabinet minister?