Devices also stop navigation devices from working

Thousands of people are using GPS jammers to disguise the fact that they are driving stolen cars, or to hide the fact that they are driving commercial vehicles for dangerously long hours and the likely consequences are more serious than your GPS screen going blank mid ride, or losing out on a KOM on a Strava segment.

Those who know say jammers create a 500m 'bubble' around a vehicle when they are plugged into the cigarette lighter, and within that area GPS devices do not work, aeroplane safety technology could be affected, and tracking systems for stolen cars fail to transmit their location. Lorry and taxi drivers who are monitored to prevent them driving for illegally long hours can also use them to avoid detection, although the report doesn't make clear how as a GPS records location rather than time spent driving.

Despite all these risks, the devices are completely legal and are sold widely.

According to The Guardian, 'When engineers began monitoring traffic on a dual carriageway outside London, and compared it with traffic on roads inside the City of London, they discovered regular use of jammers, with 10 incidents per day by some roads. That would translate to thousands of users around the country, given the amount of traffic on the road.'

Bob Cockshott, of the ICT Knowledge Transfer Network, said: "People are using these because they don't want to be detected. It's very easy to drown out the signal from a GPS satellite – which at ground level puts out as much power as a 20-watt lightbulb 12,000 miles away."

"When people use these, it creates a bubble around their vehicle for about 500 metres that jams any GPS receiver or transmitter," Prof Charles Curry of Chronos Technology told the Guardian. "It stops any tracking system the owner might have put on the car. Usually they will block GSM [mobile phone] signals too that might also be used to send back a location.

"It means that for anyone trying to track the vehicle, it just vanishes off the map – it's as though it were in an underground car park," Curry added.

Some Russian-built jammers used by the North Koreans are said to be capable of blocking signals for up to 100km and were used to block GPS traffic around two South Korean airports last year.

After an unpromising start, having to be bribed by her parents to learn to ride without stabilisers, Sarah became rather keener on cycling in her university years, and was eventually persuaded to upgrade to proper road cycling by the prospect of a shiny red Italian bike, which she promptly destroyed by trapping a pair of knickers in the rear derailleur. Sarah writes about about cycling every weekend on road.cc.