Jean Charles de Menezes' death was a tragedy that needn't have happened. The ongoing fight against terrorism meant police officers were under enormous pressure on the day he was shot, and their work in capturing the real 21 July bombers in the subsequent days is a reminder of that.

But this court case has revealed a succession of operational errors in the hours leading up to Jean Charles' death, and when something serious goes wrong in an organisation, someone must be held to account.

Sir Ian Blair's position is now untenable, and he must resign. Not because he was personally culpable, but because he was politically accountable for the work of his organisation. The same principle applies here as in all organisations. I was at the forefront of calls for Charles Clarke to resign as Home Secretary for the same reason.

No single person was to blame for Jean Charles' death, but that doesn't mean that no one should take the blame. It's one of the basic responsibilities of high office. The buck stops with you.

I do not wish to detract from the good work done by Sir Ian, and the Metropolitan Police in securing our nation's capital. Their pioneering work with community policing has done a huge amount to counter both crime and the fear of crime across London. And this is not about personality. I have tremendous respect for Sir Ian, and the work he has done in his long career.

It is about accountability. The people of London – of Britain – need to know that we have a police force that will take responsibility for its actions. As John Stevens, the former commissioner, said in an interview with The Times back in September 2005: "I think the chief constable obviously has a liability. He must have, mustn't he?"

Nick Clegg is home affairs spokesman and a leadership candidate for the Liberal Democrats