Illegal radio operators in the UK are forcing their broadcasts into motorists' cars.

The pirate broadcasters have discovered how to hijack the Radio Data System (RDS) standard. This is used across Europe and South Africa and is now being adopted in Australia, China and the US.

RDS buries digital code in FM signals so that car radios can automatically seek out any local stations that are broadcasting traffic news.

When a local traffic announcement is being broadcast, a signal is sent and the car radio switches to it. When it ends, RDS takes the listener back to the station or tape or CD they were originally listening to.

But, according to New Scientist magazine, the UK's Radio Investigation Service says the pirate broadcasters have now harnessed RDS for their own ends. They appear to have used the published RDS standards to make devices that constantly transmit the signal to switch, thereby grabbing listeners.

The strong signal swamps any car radio in the pirate's coverage area, forces it to recognise the unauthorised signal and tune in to the pirate's station.

Because the pirate continually transmits the phoney flag, the radio stays tuned in until the car goes out of range or the driver switches off the RDS feature.

The radio investigators say they are now close to raiding and seizing the equipment of an RDS pirate operation. This will allow them to find out precisely how the pirates operate their listener-stealing trick. They then hope to help broadcasters around the world develop countermeasures.