His departure is a tragedy for the man, but not a disaster for the Government

Political crises extend when a disillusioned minister resigns, but Mr Blunkett is no Sir Geoffrey Howe or Norman Lamont

The only surprise is the precise timing of David Blunkett's resignation. If he had not resigned last night he would have been forced out before Christmas. The report by Sir Alan Budd into whether Mr Blunkett fast-tracked the visa of his lover's nanny would have been the fatal blow. We know that Mr Blunkett's former lover, Kimberly Quinn, had asked to speak to Sir Alan. Judging by some reports, Ms Quinn took the view that Mr Blunkett was extremely helpful in assisting her nanny.

Mr Blunkett emphatically denied any active involvement in speeding up the official process. But last night he revealed that an e-mail had got into the Home Office system relating to the visa. The message on the e-mail to the immigration service ­ "No favours, but quicker" ­ will be one that Mr Blunkett will never forget. Sir Alan might well have cleared Mr Blunkett of overt fast-tracking but he would not have been able to deliver a definitive and unambiguous judgement.

Evidently Mr Blunkett was intensely in love at the time of these private discussions about the nanny's visa. His normally sharp political antennae deserted him about the risks. I suspect that his antennae have been blunted ever since.

Two weeks ago when I spoke to him he was confident of being out of this nightmare by Christmas, cleared by Sir Alan Budd's inquiry. It was a private session but now that he has resigned I can reveal that he spoke fluently not only about his plans in the Home Office, but also of his ambitions beyond then. He spoke also with remarkable focus on his policy agenda, but I sensed then that Kimberly Quinn was uppermost in his mind, as she had been for some time. At one point he said to me, "I need the patience of Solomon in this job ... Did I say the patience of Solomon? I meant to say the patience of Job ... Solomon is Kimberly's surname".

He had fallen in love with Kimberly Quinn. This was a personal tragedy that became a political nightmare.

But Mr Blunkett's nightmare became multi-layered. As with nearly all of these ministerial sagas, this case acquired a bizarre momentum of its own. I am still mystified as to what Mr Blunkett thought he would gain by his attack on Cabinet colleagues in the biography that is published this week. The uncharacteristic lack of discipline made his position far more precarious.

As I reported on Tuesday, two of the most senior ministers, John Prescott and Jack Straw, spoke at length over the phone at the weekend venting their understandable fury at his extraordinary onslaught. He had lost the support of cabinet ministers when he needed it most.

If the Budd report had contained any ambiguities, some cabinet ministers would have been in no mood to defend him. Privately some senior ministers, not necessarily the victims of Mr Blunkett's more scathing comments, had concluded that he had to go. Perhaps they would have reached this conclusion anyway, but Mr Blunkett's guided tour of the Cabinet was a fatal error. In the event it seems that the Budd report would have contained sentences that would have finished him even if he had wholehearted cabinet support.

It was not Mr Blunkett's fault that he fell in love or that he was so in love that he wanted to help Ms Quinn in every way possible. It was entirely his responsibility that he succumbed to the charms of a biographer and told him what he thought of his cabinet colleagues. Ministers should have learnt by now. There are only two types of contemporaneous political biographies: they either cause mayhem, harming their subjects, or they are not noticed at all. Senior ministers, like the rest of us, are probably too vain to learn this lesson. It takes a lot to say "I would rather you didn't write my biography". Still, ministers would be well advised to exercise such restraint in future.

Mr Blunkett's departure leaves a gaping hole in the Government. The Queen's Speech was virtually Mr Blunkett's one-man show. He was going to play the tough guy in New Labour's pre-election repertoire. What is more most voters approved of him, according to the polls. Even in recent weeks while some in the media huffed and puffed, a majority of the electorate wanted him to stay on.

I have known him since he became local government spokesman in the late 1980s. He is a more rounded figure than the reactionary stereotype, a genuine progressive committed to eradicating poverty and building a more community-based politics. Having led Sheffield council, he also knew more about administering than most of the inexperienced cabinet that nervously came to power in 1997. But he is also highly pragmatic, seeking to please his various leaders. He was a Kinnockite and a Smithite before becoming a Blairite. He sought too hard to please Ms Quinn as well, which is evidently why he landed in trouble over the visa.

In the short term, his departure is a big setback for Tony Blair who had defended him so vehemently when this crisis erupted. Mr Blunkett delivered for him as a minister. It is also always embarrassing for a leader to protect a minister, only for the minister to resign within weeks. This has never happened to Mr Blair before. He was always careful not to commit himself in advance of other ministerial crises. John Major never fully recovered his authority after he had backed vulnerable ministers who were still forced to resign.

But Mr Blair has bounced back from traumatic resignations before. His closest cabinet ally, Peter Mandelson, has stood down twice; in both cases the resignations were highly charged theatre. On the second occasion Mr Blair appeared in the Commons looking pale and drawn as Mr Mandelson made his last appearance as Northern Ireland Secretary. But Mr Blair has extraordinary resilience and politics will soon move on.

When they happen, cabinet resignations seem overwhelmingly signifticant, but in most cases they are not. The new Home Secretary, Charles Clarke, will move in and implement the legislation in Mr Blunkett's in-tray. Mr Clarke will seek also to be as tough on crime as Mr Blunkett. Mr Blair will make sure that happens. Downing Street is rarely off the phone to the Home Office demanding new anti- crime initiatives. No doubt Mr Blunkett will be supportive on the backbenches. Political crises extend only when a disillusioned minister resigns. This is not the case now. Mr Blunkett is no Sir Geoffrey Howe or Norman Lamont.

Mr Blunkett will not be able to move on so quickly. Within a few months he has lost someone he loved and now is without the job that shaped his life. But he knew he had no choice other than resignation. Something happened in what Mr Blunkett called the Home Office "system" in relation to the visa that should not have happened. He is a big enough player to be brought back into government at some point. But as Mr Blunkett knows better than anyone political careers are wildly unpredictable.

s.richards@independent.co.uk