Walk on the wild side: William leaves today for a tough trek with Raleigh International
Photograph: Phil Noble

NOBODY will get to know Prince William better over the next 12 months than Mark Dyer. The former Welsh Guards officer has been appointed to act as William's official companion during his gap year and to organize his travels - starting with the prince's forthcoming expedition to Patagonia.

The relationship between the two men goes back several years. In 1994 the burly, red-haired Dyer was plucked from the guards to be equerry to the Prince of Wales, and during his 18-month assignment he became friendly with William. After he left Prince Charles's service, they stayed in touch, and Dyer, 34, was a frequent visitor to Eton while the prince was at school.

Dyer has told close friends he does not want any grand title in his present role and prefers to be thought of as a "companion and mentor".

It will prove a crucial relationship at a formative period in the young prince's life. Friends of Dyer say the pair have much in common. Known to his fellow officers as "Marco", Dyer has an A-level in the history of art, the subject William is to study at St Andrews University next autumn, but both men are keener on sports and the outdoor life than burying their heads in books.

The bracing air of the southernmost tip of Chile, where William is heading today to join a Raleigh International expedition, should suit them both.

The two men also share a distrust of a certain breed of palace official. "Marco doesn't like waspish types," said one friend. "But he knows how to be diplomatic - he will bite his tongue.

"He is a good choice as a mentor for William. He is straight down the line and down-to-earth, has an easy-going sense of humor and doesn't stand on ceremony. He comes from a straightforward background: he is not from the terraces, but neither is he a landed grandee."

Dyer, who captained the Welsh Guards rugby team, was born into an army family in south Wales and describes himself as a "full-on Welshman". His father, Michael, his elder brother, Simon - who is a talented sculptor - and his grandfather were all officers.

Second Lieutenant Mark William Galloway Dyer was commissioned into the 1st Battalion the Welsh Guards in 1987. The family of Tiggy Legge-Bourke, William and Harry's former nanny, were neighbors during his childhood, and the two remain close friends.

Dyer left the Welsh Guards in 1995 to join the drink retail arm of Allied Domecq as a business development manager. Earlier this year he left to become a founding partner of The Bar Management Company, which owns the Just So bar in Wandsworth, south London.

He has told friends that he is determined to make a success of his commercial ventures and is not seeking a career in the royal household.

In the past few years he has become a constant figure in William's life and is close to some of his friends. Last summer he was invited to join Charles, the young princes and Camilla Parker Bowles aboard the yacht of John Latsis, the Greek shipping magnate.

One friend of both Dyer and the prince said the best way to describe his relationship with William was as "big brother". Last year he helped to organize an African safari for William and Harry.

For much of the past month Dyer has been with William on the remote island of Rodrigues, off the coast of Mauritius in the Indian Ocean. He was William's only companion and watched the future king teach locals how to swim and play water polo.

William's 10-week trip to Chile with volunteers, known as Venturers, from a wide range of backgrounds will involve climbing snow-capped mountains, kayaking and exploring uninhabited islands. After flying to Santiago, the Chilean capital, and spending two days traveling 1,000 miles south by bus, the Raleigh International team will set up a permanent base.

Dyer is expected to leave William there after a fortnight and return to London to continue planning the prince's next expedition, due to commence after Christmas.

Sir David Checketts, who played a similar role when he became the first private secretary to Charles while he was still at Cambridge, later developed a career in the defense industry. He said last night: "I went on numerous trips abroad with the prince and I have many fond memories.

"It is a club and one stays in touch. It doesn't do your prospects any harm, put it that way."