Professor Hawking, who suffers from motor neurone disease, was able to float free, unrestricted by his paralysed muscles and his wheelchair.

The two-hour flight over the Atlantic took a series of dramatic dives, allowing the professor to experience 25-second spurts of weightlessness.

The event could be a step closer to Hawking's goal of going into space.

Space, here I come!

Prof Stephen Hawking

The modified Boeing 727 jet simulated the experience of weightlessness as it took a series of eight plunges.

"It was amazing," Prof Hawking said after the flight.

"The zero-G part was wonderful and the higher-G part was no problem. I could have gone on and on. Space, here I come!" he said.

Initially, the organisers had planned to go through between one and three zero gravity sessions.

In the end eight were completed, and Prof Hawking experienced weightlessness for about four minutes.

US firm Zero Gravity normally charges a fee of $3,750 (£1,915) for its passengers, but that fee was waived for the Cambridge physicist.

He was not given many years to live when he was diagnosed with motor neurone disease in the 1960s, aged 22.

Long-held ambition

The plane took off from the space shuttle's runway at Nasa's Kennedy Space Center in Florida. The planes used for parabolic flights are commonly referred to as "vomit comets".

During the flight, Professor Hawking's aides lifted him out of his chair and placed him gently on the floor of the plane during the sharply angled climb into the sky, in preparation for the first plunge.

The jet's interior was padded to protect the weightless fliers and equipped with cameras to record their adventure.

"We had a wonderful time. It was incredible, far beyond our expectations," said Peter Diamandis, chairman and CEO of Zero Gravity Corporation.

"The doctors felt he was in tremendous condition. His heart rate, blood pressure, oxygen levels were all normal and perfect," Mr Diamandis said.

Prof Hawking, 65, is one of the best-known theoretical physicists of his generation. Some of his groundbreaking work focuses on black holes and on the origins of the Universe.

He has at least one other motive for taking the weightless flight: he believes private space ventures are vital to reduce the cost of space tourism and make it accessible to a greater number of people.

"I think the human race doesn't have a future if it doesn't go into space," Professor Hawking earlier told the BBC News website.

He has a reservation for a sub-orbital flight with Sir Richard Branson's Virgin Galactic venture, scheduled to begin service in 2009.

In November last year, he told the BBC: "My next goal is to go into space; maybe Richard Branson will help me."

Virgin Galactic will own and operate at least five spaceships and two mother ships, and will charge £100,000 ($190,000) to carry passengers to an altitude of about 140km on sub-orbital space flights.