Sir Richard Branson has "pretty well given up hope" that his friend Steve Fossett, the American aviation pioneer and adventurer, will be found alive.

Fossett has not been seen since September 3 when he took off from a western Nevada ranch in a single-engine plane in a search for suitable sites to attempt a new land-speed record.

Since then, extensive searches have been carried out for the 63-year-old, but with no success. Teams on foot, horseback and various vehicles have all been deployed in an area of rugged terrain just north of California’s Death Valley National Park.

In an interview today on NBC's Today show, SIr Richard paid tribute to Fossett, saying he was a remarkable individual who touched the lives of those he knew. As well as being friends, the two men collaborated in various attempts to circumnavigate the earth in a balloon.

“I think the chances are that he’s no longer with us,” Sir Richard said. “I think everybody involved has pretty well given up hope, sadly.”

The entrepreneur added that Fossett’s wife, Peggy, had written to him to say she was returning to Nevada for one final search, but the likely fate of her husband was “beginning to sink in”.

Fossett had previously survived a nearly 30,000-foot plunge in a crippled balloon and a dangerous swim through the English Channel.

Sir Richard also had warm praise for his friend’s record-breaking prowess, describing him as an "incredible individual" who had set 125 world records since the age of 55.

Fossett, who made millions as a commodities broker in Chicago, also completed the 1,165-mile Iditarod sled-dog race, scaled some of the world’s best-known peaks, sailed and flew around the world, and set more than 100 aviation and distance records.

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