London Assembly members voted unanimously for the probe which will look into how Mr Johnson's advisers are selected and whether proper recruitment procedures have been carried out.

It follows the high-profile resignation of two key aides in recent weeks, including that of Deputy Mayor Ray Lewis.

Mr Lewis stepped down last week amid allegations of inappropriate behaviour and financial irregularities, having seemingly misled Mr Johnson over being a serving magistrate.

It followed the earlier resignation of James McGrath, one of his senior advisers. Mr McGrath was forced to step down after apparently remarking that Caribbean immigrants should go home if they did not like London.

The City Hall investigation into the Mayor's appointment process will be carried out by the Business Management and Administration Committee.

It will look into how Mr Johnson picked his senior policy advisers and if the proper process was followed.

The probe will also evaluate whether the Mayor was properly advised and what lessons can be learned from the recent resignations.

The inquiry was voted for following a motion proposed by Labour member John Biggs.

Mr Biggs told the Assembly: "To lose one adviser might be considered unfortunate but to lose two in two weeks looks like carelessness. Beyond the headlines there are important issues about the appointment of publicly-funded staff that need to be addressed to ensure the good management of this authority."