The Japanese firm says the watch is the world's smallest GPS device. It displays the user's latitude and longitude by degrees, minutes and seconds - as well as all the usual clock function.

The device should give a position anywhere on the planet down to an accuracy of several tens of metres. This is the usual standard for GPS machines in civilian use. Military receivers that use special encryption technology can obtain far more accurate data.

The heart of the GPS system is a network of 24 satellites orbiting the Earth at a height of 11,000 miles. Each satellite carries an atomic clock that enables distances to be calculated from the time signals take to travel across the network.

Casio, which displayed the prototype on Thursday at its headquarters in Tokyo, said the watch could be released worldwide as early as June. It would be targeted at those involved in outdoor sports. It is expected to retail at a price between 50,000 yen and 70,000 yen (£270 and £378).

It would be an addition to Casio's Protrek range of multiple feature watches.