A new sat-nav device that uses the earth's magnetic field rather than satellites for better accuracy could be just three years away.

Experts at Porton Down and the National Physical Laboratory (NPL) claim that they are between three and five years away from developing the 'quantum compass' that will be more accurate than current devices.

The device could also be developed later for smartphones, while the military has also shown particular interest in the technology with the Ministry of Defence spending £270m on the programme.

It would mean there is no longer need for satellites or fixed point radio masts, which current sat-navs depend on. Instead it would work purely from the earth's magnetic field.

According to experts, Global Positioning Systems (GPS) currently used are at risk of becoming outdated as they are increasingly vulnerable to attack and disruption. The irregular activity of the sun, for example, which emits varying levels of radiation, can cause problems with sat-navs.

The military is particularly keen on using it for submarines, as their locations can often be out by 1km when they resurface.