The first Britain to Australia bus service leaves tomorrow with every seat sold and two Scottish travellers booked on to the second bus next week.

Passengers have been clamouring to take the "green option" to get to Australia at a cost of almost £4000.

The bus leaves London at 9am and will go on to visit 20 countries over 12 weeks.

The youngest passenger on the historic first journey is just 18 and the oldest almost 70.

A second bus - which has also sold out - leaves next week, with the passengers including Viv Wickham, 31, and Fiona Blacklock, 26, from Edinburgh.

The bus operators - Ozbus - have been stunned at the success of the new route with tickets selling at £3750 a time.

Mark Creasey, managing director, said: "It has been fantastic. We have sold out the first trip so we laid on a second and sold out that one as well.

"The only reason we're not looking for any more in the autumn is that the weather turns against us when you get down to Nepal and Tibet.

"So we will start up with further services in the spring. People have been very interested in taking an environmentally responsible way to get to Australia - and this is the perfect solution."

The bus can accommodate 36 passengers. The route takes it from the Embankment in London to Dover into Europe and then Asia via Iran, Pakistan, India, Nepal, China, Laos, Thailand, Malaysia, Indonesia and East Timor - 20 countries in all.

The bus creates less pollution than a 747 flight to Australia and this proved to be a big selling point with travellers.

Many of the customers on the first trips are adventurous Aussies and New Zealanders returning home following a gap year in Europe.

Ms Wickham quit her job as a training manager in the gas industry to take what she describes as "the opportunity of a lifetime".

She said: "When I first tell people I'm taking a bus to Sydney, they think I'm mad, but then they get very excited and jealous."

She first heard about Ozbus when she stumbled on their website. She is particularly relishing a visit to the Base Camp of Mount Everest in Nepal.

"There has been a lot of concern because we'll also be travelling through Iran and Pakistan. But when I looked up travel blogs my concerns faded away. Everything I've read has been positive."

The trip does involve one flight for the 460-mile leg from East Timor to Darwin.

But Mr Creasey, 38, is trying to find a ferry alternative for future trips.

Eventually he hopes to run a monthly service with return trips from Sydney.