Leading article: A foolish and dangerous mistake

The moment the pictures appeared of Assistant Commissioner Bob Quick, arriving at Downing Street with secret documents in full view, it was pretty clear that his survival in the job would be counted in hours not days. Even in an age, such as ours, where resignation has gone seriously out of fashion, his position was untenable. He would have been ill-advised to linger.

However irreproachable his record as head of counter-terrorism – and, given the Damian Green affair, that is open to question – no government can be in the position of asking its citizens to exercise vigilance if the top policeman responsible for the capital's security treats his access to confidential information so casually. It is no defence to say that this was a one-off mistake. It is from such small errors that big consequences can flow.

It may be that Mr Quick did not realise that there would be cameras outside Downing Street, or did not think that he would be recognised or considered worth filming. Then again, perhaps, he was unaware of the clarity of today's digital images, or just did not consider what might be in public view as he scooped up his papers. Any of these would be an understandable oversight – just not from an individual in his position.

The mistake alone should have been a resigning issue. That it threatened to compromise a major anti-terrorist investigation made any defence impossible. At least it was the planned police operation that was brought forward, rather than the alleged act of terrorism itself. Advancing so complex an operation, however, was fraught with risk. And, even though it appears to have passed off as planned, it terrified the students caught up in it and endangered many more lives. It also remains to be seen whether the right people were arrested and whether, if they come to trial, the evidence will stand up in court. This has not always been the case, even when the police operation has not been rushed.

What with the fall-out from the death of Ian Tomlinson and the resignation of Assistant Commissioner Quick, this has not been a good week, to put it mildly, for the Met. By resigning quickly, at least Mr Quick managed not to compound the agony.