What on earth has happened to David Blunkett? Since he first gained national prominence 25 years ago as an earnest left-wing firebrand, his name has been a byword for probity and decency. Whatever people thought of his politics, they respected his achievements in transcending his blindness, and the manner in which he conducted himself.

He gave straight answers. He seemed to understand the way the world really worked. And he knew right from wrong.

How things have changed. Today, there are those who regard him as a laughing stock; a figure to be pitied; a man whose career has spiralled downhill since the unravelling of his affair with a married woman. Worse still, he has been described as a liar and a bully.

Where once he looked at a future that held the very real possibility of becoming Prime Minister, today he is left clinging to the hope that when Gordon Brown eventually succeeds Tony Blair, he will be able to prolong a career which is threatening to go into precipitous decline.

The truth is that the David Blunkett of today is a very different man from that of even a year ago. Barely a week seems to go by without some new embarrassment, from allegations (which he has strongly denied) that he intervened to find out his son’s A-level results, to the latest imbroglio - his friendship with estate agent Sally Anderson who, at 29, is half his age.

As a single man, Blunkett is of course entitled to see whoever he wants.

The two apparently share a love of opera. But the most interesting aspect of this latest development in Mr Blunkett’s life is how he and Ms Anderson met. To anyone who knows the David Blunkett of old - down to earth, slightly dour and ill at ease in the bright lights of the big city - the fact that they were introduced at the West End nightclub, Annabel’s, is simply staggering.

As Blunkett’s biographer, I did a double take when I heard earlier this year that the Works and Pensions Minister had been frequenting, and had then joined, Annabel’s. Situated in London’s Berkeley Square, the nightclub is the exclusive preserve of the upper and moneyed classes. The word ‘Eurotrash’ could have been coined to describe its clientele.

Mr Blunkett long ago developed a liking for good food and wine. But it is one thing enjoying a glass or two of Burgundy, quite another for a Cabinet minister to spend the night in the louche surroundings of Annabel’s.

But his regular presence there is not just bizarre. It is, in reality, deeply sad. It's as if something inside him has cracked and it's affecting his very judgment.
To discover how and why he has changed, we need to look at what has happened to David Blunkett over the past 12 months.

This time last year, he was still riding high as Home Secretary. Although his summer had been dominated by revelations about his affair with Kimberly Quinn - the married publisher of the right-wing Spectator magazine - he had been treated by both the media and his political opponents with silk gloves. Not only was it widely felt that his private life should remain just that - private - there was also a general belief that he deserved some private happiness considering all he hardships he had suffered.

But everything changed in November, when allegations were made which put the relationship very much into the realms of public concern. Blunkett had, it was said, abused his position as Home Secretary to speed up the application process for a visa for Quinn's Filipina nanny.

Blunkett issued a full, unambiguous denial. By Christmas, however, his world collapsed and he was forced to resign from the Cabinet, after an official investigation which found that, contrary to Mr Blunkett’s denials, his private office had indeed been involved. And, with his resignation, his life changed utterly.

Ever since he became chairman of Sheffield City Council social services committee in 1976, there had been barely a day in David Blunkett's without full-time political responsibility - from leader of the city's council to senior Labour spokesman and then Cabinet minister. Now, although he remained an MP, he suddenly faced the prospect of empty, purposeless days - and all because of his involvement with a married woman. Blunkett’s friends worried that he would be unable to cope with the humiliation of his private life being paraded through the media and the collapse of his career.

His troubles had not been helped when my biography of him was published. While researching the book, I had conducted interviews with him during which he made a series of remarkably frank criticisms of his Cabinet colleagues. It was a sign of his confidence in his power and position that he made such reckless comments, even though I stressed to him that they would be on the record.

At a time when he needed all the support he he could get, letting it be known that he considered Culture Secretary Tessa Jowell 'weak', that Charles Clarke ought to have 'developed more' and that Patricia Hewitt did not think strategically did not help his position within the Labour Party.

At the time, I struggled to come up with an explanation for why he had given me his private views so candidly. It seemed so unlike him - so fatally lacking in the astute judgment of which he had always been the master.

It is only now, with hindsight, that I realise it was all part of a pattern of behaviour which shows that he has not, for some while, been the man he was. He has, it seems, lost his judgment and lost his bearings.

Of course, this coincided with his relationship with Kimberly Quinn and all the tragic fall-out that has resulted.

For years, he had been trapped inside a loveless marriage. He and his wife, Ruth, married young, and soon realised that they did not really love each other. By then, however, they had three children - so, for their sake, stayed together. When they did eventually divorce, the split was amicable and they have remained on good terms.

Blunkett was a wonderful father and still has a superb relationship with his sons. He was closely involved in their upbringing - even after the divorce. But he was nonetheless a profoundly lonely man, who would return from a hard day's work to an empty house and would cook his own supper. He told me once how he burned his fingers taking sardines out from under the grill.

Although he had other relationships - there was a brief fling with a civil servant in the Department for Education when he was Secretary of State - none had the potential to last, and he knew it.

But then came Kimberly Quinn. Vivacious, flirtatious and - so he thought - deeply in love with him, Blunkett believed he had met his soul mate and that they would be together for ever. She told him she did not love her husband, and Blunkett took her at face value. He was present at the birth of their son. He planned for their future together.

And then, when the relationship became public knowledge, she dropped him like a stone. He was utterly devastated - not least because she sought to deny him access to their son. Once more, he was plunged into loneliness, as painful as bereavement, but this time without the comfort blanket of political office on which he had grown so dependent to make up for the void in his private life.

As a flighty habitué of London ‘society’, who we now know had a string of other affairs (with a list of men including Simon Hoggart, the presenter of Radio 4's The News Quiz), she was as far removed from the women in Blunkett’s usual political and social circle as
one could imagine.

Admittedly, that was a large part of the attraction. Having spent so long alone, and working 16-hour days, he had been enraptured.

Suddenly, Quinn gave him a taste for a world he had never previously encountered.
As the epitome of the Annabel’s set - moneyed, glamorous and loose - she introduced him to its seductions. The old David Blunkett would, of course, have looked on such things with disdain. The new David Blunkett relished them.

But it is not just in his conduct of his private life that Blunkett has changed. Of a piece with his new behaviour is disturbing evidence of a penchant for ignoring the truth.
Blunkett’s reputation was built on his solidity and plain speaking. Where other politicians answered questions as if they had something to hide, Blunkett seemed to relish the chance to explain what he was doing and to tell it as it was. For many people, his outright denial that he had any involvement in the visa application for his son’s nanny was enough; if he had said it, it must be true.

That reputation was destroyed when Sir Alan Budd's official inquiry showed that he had indeed been involved. Blunkett claimed to have forgotten the incident. Perhaps.

But as his biographer, I found that colleagues and friends would cite his astonishing memory - the prodigious memory he used so effectively to compensate for his lack of sight - as his single most notable trait. It beggars belief that he would simply forget raising the matter of his own son’s nanny’s visa application.

In my book, I quoted critical remarks which Blunkett made to me about Lord Stevens, the former Commissioner of the Metropolitan Police. Yet when Lord Stevens published his autobiography last month, he accused Blunkett of being a liar and a bully. He also revealed that Blunkett wrote him a two-page letter when my book was published in which he apologised for all the 'rude remarks' he had allegedly made about him, claiming he had never made the comments and telling Lord Stevens that he had been a 'splendid Commissioner'.

Either David Blunkett had no memory whatsoever of speaking to me or he told the former Commissioner an outright lie. Perhaps he forgot the six hours of interviews, not to mention the days I spent with him, shadowing him at the Home Office and Labour Party Conference. Perhaps he forgot that our interviews were being recorded, as I repeatedly told him.

What was happening here was a terribly sad story: a lonely man, realising his life's ambitions have been exhausted and who has lost his personal compass - together with what used to be his prime asset - his judgment.

Indeed, David Blunkett's most powerful characteristic has always been his impeccable political judgement and timing: an ability to know with whom to ally, sensing when a tide is turning and deftly positioning himself to take advantage.

Today, however, he seems to have completely lost that touch.

As always seems to be the case with politicians, it was not the initial act which caused the crisis, but the cover-up. If Mr Blunkett had responded by confirming that he had indeed raised the matter of the nanny's visa with his officials, he would have been criticised. But most people would have understood why he would have been concerned about the treatment of his son’s nanny. Instead, he denied it, was found out, and had to go.

At a stroke, he lost his most invaluable reputation - for straight talking and probity. That can never be recovered. Nor can we ever now forget the bizarre image of him whiling away the night at Annabel’s in search of human contact to fill the void in his life.

David Blunkett was a towering figure in politics, whose extremely courageous personal story and public achievements stood as an inspirational example to us all. Today, he is just another politician, who is in danger of frittering away a gleaming reputation.

The real tragedy of his career is that there is a risk that where once he was revered, in future, he will be pitied.