John Rentoul: Blame the Prime Minister. He should not have brought him back so quickly

Indeed it showed commendable generosity of spirit for the Prime Minister to say in December that Mr Blunkett had left the Government "with your integrity intact" ­ a form of words that he so conspicuously echoed yesterday. But there was a big difference between the offence that ejected Mr Blunkett from the Cabinet last year and that which did so this time.

This time, he was undone by a fine example of journalistic endeavour on the part of my colleague Francis Elliott, Whitehall editor of The Independent on Sunday, whose sharpness in asking questions about the Ministerial Code was ruefully acknowledged by Mr Blunkett yesterday. Yet Mr Blunkett could claim that, although he had made a mistake ­ or the "same fault on three occasions arising from the same misunderstanding by me" ­ he had also done nothing wrong.

Provided we accepted his definition of the word wrong. "When I say I've done nothing wrong," he said, "I mean that in public office I have not misused my position in any way." In that limited sense he was right, in that he broke rules designed to prevent such misuse without actually misusing his position.

That was not something he claimed last time. In December, he was undone by an independent investigator, Sir Alan Budd, who found evidence that, when Mr Blunkett was Home Secretary, his private office had interceded on behalf of his lover's nanny to speed up her application to remain in the country.

An e-mail from within the Home Office to his private secretary read: "Sorted ­ she has been granted indefinite leave to remain ­ papers will be sent to her shortly. The case was in the Initial Consideration Unit, so they pulled it out of the queue and made a decision (no special favours, only what they would normally do ­ but a bit quicker)." As Mr Blunkett admitted at the time, "whether or not I asked for any action to be taken is irrelevant to the inference that can be drawn".

It was not a good inference, and it was one that not only required his resignation but a period of extended internal exile. Bringing him back after less than six months was calculated to undermine confidence in politics, just as letting someone out of jail who has served a mere fraction of the sentence handed down by the court undermines confidence in the criminal justice system.

This is not merely a matter of being "very careful ... that we are purer than pure", to quote the Prime Minister's precise words from 1998, but of taking care to be more brutal than brutal.

There was, to be harsh, no need to bring Mr Blunkett back into government earlier this year. On the contrary, there were good reasons of practical statecraft for not doing so. A prime minister needs to promote new talent, to maintain an upward sense of momentum for Blairite ministers below Cabinet level, and cannot afford to waste valuable places at the top table. Especially as Mr Blunkett would not have been a threat to him from the back benches, unlike many ex-ministers.

The idea that Mr Blunkett was a "big hitter" needed by Mr Blair in order to take on Gordon Brown at one of the critical reforming departments was negated by the fact that the former Home Secretary had offended too many of his Cabinet colleagues and alienated too many Labour backbenchers. It is no use being able to stand up to the Chancellor in verbal argument if you do not have the troops.

There is no friendship at the top, as one of Mr Blair's more worldly predecessors said, and it is quite surprising, and to his credit as a human being, that the Prime Minister does allow sentiment to interfere with judgement. Mr Blunkett's premature rehabilition was not his first offence. He had shown a similar quality of loyalty to Peter Mandelson, bringing him back to the Cabinet after a slightly longer but still lenient 10-month spell in the sin bin.

What Mr Mandelson and Mr Blunkett had in common was that journalists had an added incentive to go after them on ethics questions, not least because the standard required of them, after one offence, was higher than it might otherwise be. That is the other consideration of cynical politics that Mr Blair seems to have ignored. The second time round, a Cabinet minister who has been forced to resign once for doing something wrong is vulnerable because people are on the lookout for repeat offences.

This is nothing to do with Mr Blunkett's complicated private life. The moment when I thought his fate was sealed was when Stephen Pollard, his once sympathetic biographer, called him a liar. Mr Pollard had quoted disparaging comments that Mr Blunkett had made, on the record, about Sir John Stevens, the former Metropolitan Police commissioner. Two months <BR>

ago, Sir John in his memoir said Mr Blunkett wrote to him denying that he had made the comments and saying that he thought he had been a splendid commissioner.

The Prime Minister's judgement has therefore been called into question. Not by sticking by Mr Blunkett for so long, or even by saying that he wanted him to stay at his post, accepting the resignation only "reluctantly". Nor by insisting that this breach of the Ministerial Code was not one that required resignation.

That might have been true had the offence been committed by an unknown quantity arriving in the Cabinet for the first time with an otherwise clean licence. Mr Blair's judgement has been found wanting in reappointing Mr Blunkett to the Cabinet in the first place. To borrow Mr Blunkett's phrase from yesterday, the Prime Minister should have been able to "smell and feel" trouble.

It is often said that Mr Blair is ruthless, and that he is too much of a politician. His mistake on 6 May this year was that he was not ruthless enough, and not enough of a politician. He should have left Mr Blunkett where he was, and now is again.

What was most interesting about yesterday, however, was the identity of the one politician who made this point. Mr Blunkett's fall "does reflect badly on the Prime Minister's judgement", he said, because it was the second time he brought back a Cabinet ally too hastily, and for the second time found his confidence repaid in the form of a humiliating second exit. Who said that? None other than the shadow Education Secretary and next leader of the Conservative Party, David Cameron.

j.rentoul@independent.co.uk

The writer is chief political commentator for 'The Independent on Sunday'