How does the breathalyser that keeps cars locked work?

Road safety campaign group Brake is calling for convicted drink drivers to have breath-test immobilisers fitted in their cars when they are allowed to drive again.

The breathalysers are connected to the car's ignition and will not start the car unless the driver gives a negative test.

The distributor of Alcolock, Concateno, says some bus and lorry fleets already have the gadgets fitted and suggests that parents of young drivers should consider installing it too.

BBC Radio 5 live Breakfast's Stephen Chittenden tried it out.