A hundred nights in a tent under the stars with Ewan McGregor. Tempting? Not if every night with the Scot is accompanied by the threat of attack from Russian bandits or Alaskan bears. And not if every day involves riding the equivalent of London to Edinburgh on a bucking motorbike laden with the equipment necessary to keep man and machine alive for three months. Then there are the washing facilities. The Hollywood hearthrob’s tent might not be the most wholesome place after a few days.

“One shower a week if we are lucky,” predicts McGregor, stomping up and down in a loft room above a motorcycle garage in a backstreet in west London. “Hopefully we will smell so bad bears won’t want to come anywhere near us.”

In the next few days, probably while the capital sleeps (“in case we embarrassingly forget something and have to turn back”), McGregor and his best friend and fellow actor Charley Boorman will kiss their wives and daughters goodbye, steal out of their London mansions, and climb aboard identical shiny silver BMW off-road bikes. Then they will ride around the world to New York.

“I can’t wait to f****** get on the bikes and get out there riding,” says McGregor, who already looks the part, clad in a black biker jacket, heavy black belt, oil-splattered jeans and scuffed steel-capped black boots. “It’s been a long and occasionally tedious time planning.”

McGregor’s backdrop as he paces the loft is a 12ft-wide map of the planet. Pinned onto it is a ribbon marking the route of the most risky motoring adventure undertaken by a Hollywood star since Steve McQueen and Paul Newman raced at Le Mans.

Walking the length of the ribbon — which runs from London through eastern Europe, across Kazakhstan, through Siberia, across the Bering Sea to Alaska and on another 4,000 miles to the Atlantic Ocean — takes McGregor six steps. Each pace equates to a fortnight on an expedition he and Boorman have christened The Long Way Round.

This is no PR stunt. It is 18,000 miles long. It will take them across 19 time zones and three continents, over the Ural, Tienshan and Rocky mountains and through two deserts. It should have been named The Bloody Long Way Round.

“This trip’s just grown out of daydreaming. Originally we were thinking about going to China, because my wife was brought up in China,” says McGregor, who has finally stopped pacing and sat down. “At that time we thought our wives and kids will fly out somewhere and we’ll ride to meet them, spend a couple of weeks on holiday with them, and ride back. But that’s still a very long way away. So then we thought, let’s ride right round the world.” McGregor and Boorman deny this is a midlife crisis, describing it as a thirtysomethings’ gap year condensed into 15 weeks.

They were going to do the trip alone, unannounced. But the temptation of a multi-million-pound book and TV deal has added significant baggage to the project. McGregor hardly needs the cash and is unsure whether it was the right thing to do to agree to a cameraman and photographer coming along on “their adventure”. But come along they will, following the bikers at a respectable distance in a caravan of Mitsubishis to document the journey.

“We thought we’d take our own cameras then thought, if we’re going to film for ourselves anyway, why don’t we get someone else to do it,” says Boorman. “And the book thing sort of sprang out of nowhere…” “. . . it’s a good book deal. It’s a big number, a big deal,” interrupts McGregor.

Even so, they are both keen to retain credibility among the biking fraternity: “I had a meeting in Oz, when I was making Star Wars, with this guy and was telling him about the trip,” says McGregor. “And he said, ‘You know, I don’t want to sit down and watch this and think, wankers!’ Well, neither do we. Bastards yes, but wankers no. There is a subtle difference.”

McGregor and Boorman first met on the set of Philippe Rousselot’s film The Serpent’s Kiss in 1996. Charley is the son of director John Boorman, starring in many of his father’s films such as Deliverance, Hope and Glory, and Excalibur. But it's their shared love of bikes that has cemented the friendship, with Charley even persuading Ewan to support his racing team in the British Superbike Championship in 1998.

McGregor himself has been into bikes since he was a teenager. Heaven is riding really fast, the actor said once — he claims he has touched 150mph. Almost five years ago he had his most celebrated crash, falling off his Honda when it skidded on a patch of diesel on a Scottish roundabout when he should have been attending the 1999 Oscar ceremony. He was saved from little more than cuts and bruises by his leathers but his reputation has since put him on a collision course with producers. While filming the Star Wars movies in Australia he rode to work on his bike. "It seems the insurance bods at the Fox Studios in Australia aren't too happy that I've been taking this form of transport," he said at the time. "I was told they would much prefer me to be like any other star and take the chauffeured limo."