The system, which is housed on a truck and trailer, consists of six cells, a reception area, and an interview room.

It is being deployed by British Transport Police as part of an anti-robbery initiative and will be used by other forces when not in use.

The first prisoners are expected to use the unit, which is at Stratford station in east London, on Thursday. They can be held there for up to six hours.

"We are bringing the cells right to the very doorsteps of the criminals," said Chief Supt Paul Crowther of British Transport Police London Underground Area.

"There will be no hiding place for the vandals, touts, drunks and other anti-social elements."

Suspects can be held for a maximum of six hours before they have to be transferred to a police station or released.