Commentary: No more exits, now Mayor must show he can govern

This is the third senior resignation from the new Mayor's regime, which now looks a bit like one of those Agatha Christie novels where guests are invited to a party and then vanish one by one. Political adviser James McGrath quit because of remarks he made about Caribbean people, while former deputy mayor Ray Lewis resigned over allegations about his CV and past conduct. Tim Parker's departure within weeks of his appointment leaves the Mayor open to accusations of inexperience - at the very least.

The pity is that Mr Johnson's intentions in appointing Mr Parker were right: the GLA's agencies needed an efficiency drive. Under the high-spending Ken administration numbers employed boomed between 2000 and 2008 to the point where City Hall was splitting at the seams. The London Development Agency's new chief, Peter Rogers, has been cleaning out the stables. More of this approach is needed.

Who will benefit from Mr Parker's untimely demise? The most obvious winner is Sir Simon Milton, recently appointed as deputy mayor for policy and planning. His arrival at City Hall had led to speculation of a power struggle between him and Mr Parker. Sir Simon has experience both of leading Westminster council and, at national level, as head of the Local Government Association. Former Tory mayoral candidate and former transport minister Steve Norris should also be a winner. His recent performance at the TfL board was widely seen as Olympic gold standard. He was on top of the paperwork and the issues.

Before today's resignation Mr Parker was to chair TfL, with Mr Norris as a senior board member. Now the transport firmament will be dominated by the massively experienced Mr Norris, even if the Mayor chairs TfL. It begs the question: why not put Mr Norris in charge?

The Parker resignation comes at a bad time. Not only does the Boris regime appear disoriented, but the police's personality wars make New Scotland Yard look unstable. Allegations are flying around which damage all of London's policing. Someone needs to knock heads together at the Met - and the Mayor, with his massive mandate, is best placed to do it.

His remaining deputies are all experienced local government people. The Mayor himself is an old, if eccentric, hand at the national level. There must be no more resignations. A clear picture of what is to be achieved must emerge.

Mr Johnson's summer honeymoon is fast ending. The autumn will decide whether he can really govern the city.