Senior figures in Boris Johnson's administration believe that Kulveer Ranger should be removed from his position as Director for Transport Policy.
Ranger is seen by some in the Mayor's team as 'over-promoted' and incapable of pushing through the policies promised in the manifesto.
But with the resignations of three of the Mayor's other advisors still fresh in the memory, some believe that it would be just too damaging for anyone else to be forced from their job.
When Ranger was first appointed, it was widely reported that he had been in charge of implementing the Oyster card.
But with that claim now discredited, some members of the Mayor's senior team believe that Kulveer is simply not up to the job.
Of course today's revelation is likely to be denied by the Mayor and may only serve to strengthen Kulveer's position at City Hall.
And if he really is up to the job as his allies insist, then he should not allow his colleagues to plot against him any further.
But with other GLA jobs currently under threat, and with transport policy having such a big effect on our daily lives, it is hugely important that the Mayor should be seen to have confidence in his man.