The test, on 200 buses along 19 routes, involves 'contactless' smart photocards which passengers briefly place on a card-reader machine as they board. A green light and a single tone will indicate that the card is valid. A red light and a continuous tone indicates an invalid card, bringing it to the attention of the driver. The cards do not even need to be taken out of wallets for them to be electronically read.
The first stage of the test involves only cards valid for one week or one month, but soon 'stored value' smart cards - that work similar to phonecards - will be available for pounds 10.
The smart cards are only available from Harrow bus station in north-west London, in exchange for valid travel photocards, but can be updated at newsagents and some Underground stations.
Philip Carter, a London Transport spokesman said: 'The cards at the moment cost pounds 5 to produce, but that will be reduced to around pounds 1 if they are introduced throughout the system. They each have two microchips giving them as much computing capacity as the small personal computers of the early 1980s.'
London Transport expects that between 25,000 and 30,000 of the free cards which include a photograph of the holder, will be issued over the next few weeks. To install the equipment throughout London Transport buses would cost pounds 33m. The cards could not be introduced to London Underground without spending an extra pounds 17m to adapt the existing magnetic-strip ticket system.