The resignation of another key aide to Mayor Boris Johnson has sparked renewed questions over the Mayor of London’s leadership, with opposition leaders at City Hall charging that the “wheels are coming off” his new administration.

Tim Parker , the First Deputy Mayor and Chairman of Transport for London (TfL), has stepped down from both jobs, saying it was inappropriate for him to hold them as an unelected official. His resignation is the third of a key aide in the four months of Johnson’s mayorship.

Despite having a rather big job to do already in running the capital, Johnson will now chair TfL himself. In his usual combative style he announced: “Over the last few weeks, it has become increasingly apparent to both of us that the nature of the decisions that need to be taken are highly political and there is no substitute for me, as the directly elected Mayor, being in charge.There are limits, therefore, to what can be delegated.”

It begs the question why Parker was installed in both roles in the first place - the running of London’s transport system has always been a politically charged issue.

More important for the future though is whether Johnson is biting off more than he can chew by chairing TfL. Even in times of crisis, delegation to top aides will be crucial for the mayor’s success. One of the key arguments against voting for Johnson as mayor was his perceived lack of managing a large organisation, and judging by the rate at which senior aides are disappearing, he looks set for a rough ride, not least as yet another strike is looming on the London Underground.