The prince is said to be in Kenya for the wedding of close friend Jecca Craig's brother Batian.

A Clarence House spokesman declined to comment on the trip today, saying only that William was unable to attend his cousin's wedding due to a long-standing prior private commitment.

Mr Craig will reportedly marry his English bride Melissa Duveen in an outdoor Masai-themed ceremony.

William has known the Craig family for many years and is said to pay regular visits to their Kenyan home. Miss Craig has often been described as a former girlfriend of the prince.

Miss Middleton's attendance at Peter Phillips's wedding in his place will send a clear sign of her acceptance into the Royal Family.

Prince Harry's girlfriend Chelsy Davy will also reportedly be at the wedding, in St George's Chapel at Windsor Castle, and will be formally introduced to the Queen.

The Prince of Wales and Duchess of Cornwall, Prince Harry and the Duke of Edinburgh will also be in attendance to see Mr Phillips and Autumn Kelly, both 30, exchange vows.

They met in 2003 at the Montreal Grand Prix, where the couple were working. But he did not he was the Queen's eldest grandson and 11th in line to the throne, a fact she discovered only when she spotted him in a programme about Prince William.

Mr Phillips, the first of the Queen's grandchildren to marry, proposed last July during a rain shower as they walked the dog after bad weather forced him to abandon plans to pop the quest-ion during a hot-air balloon ride.

It emerged recently that Miss Kelly, a former Roman Catholic, has been welcomed into the Church of England.

If she had not changed churches Mr Phillips would have had to give up his right to become king. Under the Act of Settlement of 1701 heirs to the throne who marry Catholics cannot become sovereigns.

Margaret Hammond, the Princess Royal's former personal assistant, came out of retirement to organise the wedding day.

Party planner to the stars Peregrine Armstrong-Jones, who was behind Elton John's 40th birthday bash, the Beckhams' nuptials and the 21st birthday parties of Mr Phillips and his sister Zara, is organising the post-wedding celebrations. The reception and dance take place at Windsor's Frogmore House.