Critical infrastructure and other technology dependent on global positioning satellites are increasingly threatened by attack from widely available equipment, technology experts are warning.

Air traffic communications, the electricity grid, telecom networks and other emergency services are under threat from GPS jammer devices that are available cheaply in underground market places.

A portable jammer in a tall building could cover most of London and aircraft approaching its airports, said Professor David Last, a past president of the Royal Institute of Navigation and now a GPS consultant.

Experts met in UK and at the National Physical Laboratory in Teddington on Tuesday.

GPS gives us transportation, distribution industry, 'just-in-time' manufacturing, emergency services operations - even mining, road building and farming, all these and a zillion more, Last said.

But what few people outside this community recognise is the high-precision timing that GPS provides to keep our telephone networks, the internet, banking transactions and even our power grid online.

As reliance on GPS increases, more organizations are becoming vulnerable to interference. Some could be unintentional, such as solar flares or malfunctions, but some could be criminal.

Jamming devices that can disrupt GPS signals are sold on the Internet for around $100.

As an example, Last said they could be used by thieves to block transmissions from stolen vehicles protected by GPS tracking systems.

These devices are selling; terrorists have been arrested with them, he told the symposium. GPS now is like computers before viruses. But there are no big security companies working to protect GPS.

In another example Last explained that the gadget could be used to spoof location -- useful to evade GPS based tolls or to set others off course.

You can now buy a low-cost simulator and link it to Google Earth, put on a route and it will simulate that route to the timing that you specify, said Professor Last.

A GPS receiver overcome by it will behave as if you're travelling along that route.

Jamming and spoofing are irresistible to the hacker type who do it for fun, Last said.

The Royal Institute of Navigation stresses the importance of developing a ground-based back-up to global navigation satellite system.

Navigation is no longer about how to measure where you are accurately - that's easy, Last said. Now it's all about how to do so reliably, safely and robustly.