Image caption The hydrogen powered plane will be tested in 2011

Boeing has unveiled its unmanned hydrogen-powered spy plane which can fly non-stop for up to four days.

The high-altitude plane, called Phantom Eye, will remain aloft at 20,000m (65,000ft), according to the company.

The demonstrator will be shipped to Nasa's Dryden Flight Research Center in California later this summer to prepare for its first flight in early 2011.

Boeing says the aircraft could eventually carry out "persistent intelligence and surveillance".

It isn't built for stealth - it's built for endurance Chris Haddox, Boeing Phantom Works

It is a product of the company's secretive Phantom Works research and development arm.

Boeing says the aircraft is capable of long endurance flights because of its "lighter" and "more powerful" hydrogen fuel system.

"We flew Condor [the company's previous reconnaissance drone] for 60 hours in 1989 on regular jet fuel, and that was the maximum," said Chris Haddox from Boeing Phantom Works. "Now we're talking 96 hours."

The company explained in a statement that Phantom Eye was "powered by two 2.3 litre, four-cylinder engines that provide 150 horsepower each".

It is also very large, with a 46m (150ft) wingspan.

"It isn't built for stealth - it's built for endurance," Mr Haddox told BBC News.

The UK Ministry of Defence (MoD) has an ongoing interest in long-endurance high-altitude planes for surveillance and is considering a several different technologies, including solar power, to meet the requirements of what it refers to as its "Scavenger project".

The aerospace and defence company Qinetiq are carrying out trials in conjunction with the MoD to develop a solar powered plane called Zephyr.

A spokesperson for the MoD said: "Four days is very good but we are considering a range of options for our deep and persistent reconnaissance requirements.

"Some of these options could be airborne for over a week."