Branson's new stunt: around the world in 80 hours, non-stop

Between them, they have been stranded in the Atlantic, plunged twice into the Pacific, caught fire several miles up and been dragged upside-down for three miles across the Australian outback.

But the past ballooning and boating experiences of Richard Branson and the American billionaire Steve Fossett appeared to have done little yesterday to dissuade them from further exposure to extreme danger - and lucrative publicity - as they announced their latest attempt at an aviation first.

The two globetrotting entrepreneurs revealed the £1.5m ultra-light aircraft that they hope will set the record for the only non-stop solo flight around the world - and generate about £60m in free advertising for Sir Richard's Virgin Atlantic airline in the process.

Speaking at a glitzy launch among the space travel displays at the Science Museum in London, Sir Richard, 53, said: "This is the last great aviation record left inside the Earth's atmosphere. To fly planes, boats or balloons around the world you have to be a bit insane. Steve and I both have the attitude that you have to live life to the full."

The Virgin Atlantic Global Flyer, a vast flying wing fashioned from the latest in hi-tech composite materials, will attempt the 25,000-mile journey next spring in 80 hours using just nine tonnes of fuel.

Barring a serious last-minute injury to Mr Fossett, who flew with Sir Richard in his failed 1998 balloon circumnavigation attempt, it is also a record bid that the British billionaire will make with his feet remaining firmly on the ground.

The 59-year-old American adventurer, who completed the first solo circumnavigation in a balloon last year, will be the first-choice pilot, with Sir Richard acting as his back-up.

The team intends to take off from a two-mile airstrip in the United States and then use jet streams at an altitude of up to 52,000ft to follow a route via London, Rome, Cairo, Karachi, Shanghai, Tokyo and Los Angeles.

Mr Fossett, whose 7ft cockpit will allow him to lie down, will have to stay awake throughout the flight, despite the fact that Global Flyer will be fitted with an autopilot.

Dressed in a Virgin red flying suit, Mr Fossett said: "This is an experimental aircraft. There is a considerable risk that the aircraft could fail in turbulence."

Organisers of the record bid were at pains to underline the environmental merits of Global Flyer, which is being built in the Mojave Desert of California and will have its first test flight to coincide with the centenary of the first manned flight on 17 December.

The aircraft will consume less than one-hundredth of the amount of fuel a conventional airliner would take to cover the same distance. Sir Richard said he believed the composite technology used in the plane would eventually form a big part of the materials used to build commercial airliners.

The billionaire, whose airline is covering most of the $3.5m (£2.2m) cost of the project, was also disarmingly frank about the more immediate benefits for his own company, admitting: "We certainly get our money back from a project like this."

The airline boss, who admitted the announcement of Global Flyer was timed to coincide with the last flight of Concorde, also could not resist a swipe at both British Airways and the Government for failing to keep the gas-guzzling aircraft in the skies.

He said: "Concorde is capable of flying for another 20 or 30 years. It was built by British and French taxpayers - we should all make an enormous effort to make sure that future generations can actually see Concorde fly and not in a museum."