Security experts are warning that mischievous hackers could send bogus signals to satellite navigation systems via the radio.

The problem centres on the Radio Data System, which allows FM broadcasters to include data in radio transmissions that many satnav systems pick up.

The data is commonly used to give traffic or station ID information, but this can now be hijacked, according to researchers.

Andrea Barisani, chief security engineer at consultancy Inverse Path, and Daniele Bianco, a hardware hacker at the same company, built a system that can generate false warnings for drivers using off-the-shelf components.

The messages range from standard traffic alerts to warnings of an "exploding bomb ahead" or that the road is "blocked by a bullfight".

Almost any programmed message built into the satnav system can be activated because the data stream does not need to be encrypted or authenticated.

The researchers advise drivers not to react instantly if they receive a highly unusual message but to check with the news services before taking action.