London Underground is investigating the incident

He was allegedly seen watching a film on a portable player in the cab of a passenger train, alongside the trainee.

The instructor is not thought to have been driving the train. He has been suspended on full pay while an internal investigation is held.

LU has refused to give any further details of the alleged incident.

But an LU spokesman said: "When the performance of any staff member is called into question, we undertake a full investigation to establish the facts...where appropriate, disciplinary action is taken."

The incident is alleged to have taken place on the Northern Line, one of the busiest on the Tube system.