For the third time in seven days, Sir Ian Blair insisted today that he would not resign as Commissioner of the Metropolitan Police.

With the backing of the Home Secretary — also expressed three times in the past week — it seems certain that he will now remain in post.

But whether Sir Ian has been, as he claimed, “entrusted with the long-term development of the Metropolitan Police” is yet to be seen.

The suggestion in well-informed corners of Whitehall is that the Commissioner is being allowed to continue in post at least in part because there is no obvious successor to step into his shoes.

Having backed him again and again and again, Jacqui Smith has put her political credibility on the line.

David Davis is said to have a 100 per cent record in witnessing the demise of those whose resignations he has demanded. But this time it looks as though he will be denied his scalp.

Sir Ian will soldier on, but his every action will be scrutinised. Should he put a foot out of line, his critics will pounce and ministers will get nervous.

At the end of the Stockwell trial Mr Justice Henriques criticised the “entrenched position” adopted by Scotland Yard and, over the past week, the Commissioner has sounded peeved and angered. But when he spoke today, Sir Ian adopted a more contrite tone.

“Hardly a day goes by when I have not thought about how things could have been done differently and thus Mr de Menezes would still have been alive,” he said.

He apologised once more to the de Menezes family, talking of his “deep regret”, but combined that with expressing full support for his officers some whom have been praised for showing exemplary courage on July 22, 2005.

Sir Ian also responded to direct criticism of his decision to block the IPCC's investigation for five days by saying that he had made a mistake and would act differently if a similar situation were to arise today.

It was a sign perhaps that he believes the worst week of his career has passed. The de Menezes saga is not going away - the Metropolitan Police Authority has to debate it and there will be an inquest and further courtroom battles. But it has probably lost its power to damage the Commissioner.

Sir Ian will hope to return to the tasks that make people (and voters) feel safe: continuing to cut crime, tackling the problem of teenage violence, building on the successes of neighbourhood policing.

But despite his talk of shaping the Met's long-term future, he knows that any further damage to his reputation, and that of his force, is unlikely to be tolerated by his political masters.

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