London mayor Ken Livingstone is facing his first threat of industrial action on London Underground since he took control of it.
Train drivers are being balloted
Drivers on the Tube's Metropolitan Line are to be balloted over a number of issues including the dismissal of a colleague for opening Tube doors on the wrong side.
Officials from the drivers' union Aslef complained that the sacked man was treated differently from other drivers who had committed the same mistake.
When Mr Livingstone took over the Tube he said he hoped to agree a four-year pay deal with Tube workers to end the threat of strikes.
He has already pledged to open negotiations with union leaders on this matter.
Mr Livingstone's Transport for London (TfL) took over the Tube last week and now has control over how the network is managed and run.
A new London underground management team is in charge of day-to-day operations and staffing of stations, while two infrastructure companies (Infracos) are responsible for maintenance and modernisation for the next 30 years.