A Metropolitan Police firearms officer is under investigation after a Taser was lost when he drove off with it still on the roof of a police vehicle.
The weapon was left on top of the vehicle as the officer left a briefing at a force firearms base in Norfolk Row, Lambeth, between 0700 GMT and 0830 GMT on Tuesday.
Officers said the stun gun and four cartridges were left on the roof.
Police realised it was missing at about 1000 GMT.
The Taser is believed to have fallen off somewhere between Norfolk Row and Vauxhall Cross, where the officer had stopped to refuel.
Scotland Yard said the officer concerned had been taken off operational firearms duties.
A spokesperson said the Directorate of Professional Standards had been informed and an internal investigation was under way.
Police urged anyone who found the Taser, which is shaped like a handgun but made from bright yellow plastic, not to touch it but to contact the force.