US officials are worried that the 20-year-old global positioning system (GPS) that sat-navs and other navigation devices rely on could begin to fail by next year.

A report from the US Government Accountability Office (GAO), said that a lack of investment in satellites means that some are close to failing.

The GPS network is run by the US military and used by a number of commercial firms internationally for sat-nav systems.

"It is uncertain whether the Air Force will be able to acquire new satellites in time to maintain current GPS service without interruption," says the GAO report.

"If not, some military operations and some civilian users could be adversely affected."

The only other such network in place today is the Glonass Russian satellite system, which currently has only 19 operational satellites, with 24 needed to cover the whole globe.

The US Air Force is spending £1.3bn to bring the US system up to date, but the GAO report says that repairs are not happening fast enough.

The first replacement system should have been launched at the beginning of 2007, but has been delayed several times and is now scheduled for November this year.

The US system currently has 31 satellites operating, but a number of them are more than four years past their design lifetime.

The GAO predicts a one in five chance that the fleet will drop below 24 at times in 2011 and 2012.

The news will provide further ammunition for advocates of the EU Galileo system, which they claim will provide Europe with its own secure non-military system. It is expected that four satellites within the network will be operating next year.

But critics, including the House of Commons transport committee, described the system as unnecessary and overly expensive, labelling it as an "orbiting railtrack".

China also hopes to have its own GPS system operational by 2011.