Both actors have undertaken extensive training for the trip

The Scottish film star, most recently seen in Tim Burton's Big Fish, will be joined by fellow actor Charley Boorman, son of Excalibur director John Boorman.

The journey, beginning in London later this week, crosses Eastern Europe through Siberia, across the Bering Sea to Alaska and on to the Atlantic Ocean.

McGregor, 33, whose adventures will be filmed for a future documentary, called it "a once-in-a-lifetime trip".

SAS training

McGregor said the pair had been preparing for the three-month trip since January.

Both actors have undertaken hostile environment training with a team of ex-SAS soldiers, and learned how to perform emergency medical treatment, in case of a serious accident.

"When we are in places like Siberia we will be days away from any medical assistance so the medical training was vital," said McGregor.

Their journey will take them through some of the world's most inhospitable terrain, including Kazakhstan, Mongolia and Siberia.

Bare necessities

The pair plan to carry all their own equipment, and will live off a basic diet of rice, lentils and dehydrated camping food, supplemented by freshly caught fish.

I'm looking forward to it and can't wait to get on the road Ewan McGregor

They will camp where necessary and stay in local hotels where possible.

"If you ask anyone that goes on adventures it's very difficult for them to explain why. Maybe the better question is 'why not'," said McGregor.

"Riding in severe weather and not being able to have a bath at the end of it will be tough," he said.

"But other than that I'm looking forward to it and can't wait to get on the road."

Both men, who have been friends since meeting on the set of the film Serpent's Kiss in 1996, are married with young children.

Lifetime trip

Asked what his family thought about the journey, McGregor said: "I think if the whole thing was called off tomorrow that they would probably be delighted."

"But... they're fully behind us, it's a once in a lifetime trip and they all understand that."

"It is going to be so hard not seeing them for this length of time but we are taking a mobile and a satellite phone and we will ring home every day."

Boorman said he was confident the trip would be a success.