The couple will exchange vows in the historic St George's Chapel at Windsor Castle as Peter's mother the Princess Royal, the Queen and other senior royals look on.

Among the wedding party will be the Prince of Wales and the Duchess of Cornwall, Prince Harry and the Duke of Edinburgh. Prince William will not be attending as he has a prior engagement.

Peter and his Canadian bride-to-be met in 2003 at the Montreal Grand Prix where the couple, both 30, were working.

But the royal did not tell his future fiancee that he was the Queen's eldest grandson and 11th in line to the throne.

The Canadian has revealed she only discovered who he really was a number of weeks later when she spotted him in a programme about Prince William, in the run-up to his 21st birthday, and that it "scared" her.

The Queen's grandson proposed to his fiancee last July during a rain shower as they walked the dog because the bad weather had forced him to abandon plans to pop the question during a hot-air balloon ride.

Miss Kelly said: "He went down on one knee in the downpour and produced a ring from his pocket. I certainly didn't see his question coming on this particular day. I looked horrible, in my wellies with wet hair. I said 'Yes' straightaway."

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

If she had not changed churches, Peter, who is 11th in line to the throne, 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.