In the end, Blair was too weak to save his Home Secretary

The PM put the whole weight of his authority behind Blunkett, and has been made to look foolish

It ended just like every previous ministerial resignation. The Prime Minister expresses his total confidence in a beleaguered cabinet minister. Every cabinet minister and Labour MP interviewed by the media expresses their total support and stresses how he has done nothing wrong. But ultimately the minister becomes utterly disabled and is unable to carry out his normal duties.

This is a massive blow not only to David Blunkett but also to Tony Blair. The Prime Minister put the whole weight of his authority behind his favourite minister and has been made to look as foolish as his Home Secretary. The initial judgment of the media, the Opposition and the general political class was that if anyone could "tough it out" it would be Mr Blunkett. Any other minister ­ Peter Mandelson, Ron Davies, Geoffrey Robinson or Stephen Byers ­ would have been forced out over a fortnight ago. The Home Secretary was always able to get away with more than anyone else. His physical disability enabled him to gain respect and admiration when, on previous occasions, he dropped the odd clanger.

Mr Blunkett will be missed by Tony Blair because he was able to reach parts of the electorate ­ especially middle-class Tories worried about law and order ­ that others could not. But he was also a popular ­ as well as populist ­ Home Secretary in the eyes of those on the council estates, from which he himself originated, where crime disfigured the lives of the less well-off. His main enemies came from the chattering classes angry at his willingness to criticise judges and his all too cavalier attitudes to the traditional safeguards to democracy. Anger across the political spectrum led him to be on the receiving end of severe criticism. A growing number of Labour MPs were getting nervous at his attacks on jury trials ­ and he caused controversy wherever he went.

But the bizarre love affair with Kimberly Quinn, which began in 2001, was to overshadow what was otherwise a successful personal ministerial career. Few were aware of this emotional aspect of Mr Blunkett's life until the news surfaced in a Sunday tabloid earlier this summer. For several weeks the story was presented as a happy love affair. The situation changed when it emerged that the relationship was actually breaking up and that Mr Blunkett may have had a hand in leaking the story to the newspapers in the hope of embarrassing Mrs Quinn. Some reports suggested that this would encourage her to leave her husband for the Home Secretary.

In the end, Mr Blunkett threw caution to the wind in his determination to prove his paternity in relation to Mrs Quinn's son and her expected second child. Of itself, this remained essentially a private matter with no apparent public interest. When the relationship broke down, however, it seemed that Mrs Quinn was bent on putting her side of the story to the media. From then, it was no longer a private matter.

Why Mr Blunkett was so clear in his mind that he had nothing to do with her nanny's visa application is utterly inexplicable. He was so emphatic that he had brought no influence to bear on the application that he was even prepared to order an inquiry to investigate his own behaviour. But the fact that he has resigned before the Alan Budd inquiry has reported indicates that not everything will appear to the public quite as Mr Blunkett and Mr Blair suggested.

It is a sign of Mr Blair's growing weakness as a Prime Minister that he has been utterly powerless to save his Home Secretary. To put so much personal authority behind such a pivotal figure in the Government only to lose him a fortnight later, suggests the long-term fallout from this event will have consequences far beyond the immediate future. In John Prescott's words, used in an earlier crisis for the Prime Minister, the tectonic plates are truly shifting for Mr Blair. In the end, this resignation is as much a cruel reminder to the Prime Minister that the support of the Cabinet and backbench MPs still matters.

A beleaguered minister needs all the support he can get. At first Mr Blunkett had considerable political capital upon which to draw. But three weeks of relentlessly damaging headlines have taken their toll, and this support was ebbing away, notwithstanding the hope that the Budd report would exonerate him. The fatal blow was probably this week's publication of a biography of Mr Blunkett laying bare his robust comments about the likes of Jack Straw, Charles Clarke and Tessa Jowell.

Ultimately, it has become clear that Mr Blunkett misused his ministerial position, and he has paid the inevitable price. When authority ebbs to that extent, even the strongest minister cannot withstand the whirlwind. His opposite number, the Tory home affairs spokesman David Davis, has his second ministerial scalp (the other being Beverley Hughes last March), although he was careful not to crow. In some respects, Mr Davis would have preferred a slow, lingering political death. And Charles Clarke will certainly be no pushover.

mrbrown@pimlico.freeserve.co.uk