Transport for London (TfL) is paying £10m for staff to work for transport unions under London Mayor Sadiq Khan, as strike days cause chaos for commuters.

The annual cost has doubled since 2015/16, the last year Boris Johnson was mayor. That year 699 members of TfL took paid time off to do union duties, at a cost of £4.4m.

Two years later, this figure had more than doubled to £10.8m, paid to 731 workers, 37 of whom worked for unions full-time.

The exact amount of union work funded by TfL is unknown as the organisation only reports the total number of staff undertaking union duties and the number working more than half of the work week or full-time.

This increase in union funding in 2017/18 coincided with a budget deficit of nearly £1bn.

Mr Khan is the chairman of TfL and the chair of its board. He is responsible for setting out strategy and holding its Commissioner Mike Brown to account.

Strikes have continued since Mr Khan took control of City Hall, including several this year. In April, there was chaos on the District line after Aslef called a strike over the sacking of a tube driver for running through three red lights.