Britain's Prince William attends the Colonel's Review ceremony at Horse Guards Parade in London June 4, 2011. The annual military parade and march-past by the British Army Household Division takes place a week before the Trooping of the Colour which marks Queen Elizabeth's official birthday. [Photo/Agencies]

LONDON - Sharp-eyed tourists outside Buckingham Palace had a rare opportunity Saturday to see Prince William on horseback.

But even those in the vicinity might well have missed him, since his well-known face was obscured by a traditional tall bearskin hat.

The prince was taking part in a rehearsal for next week's "Trooping the Colour," a major event to mark Queen Elizabeth II's official birthday.

The queen turned 85 in April, but the public party for the monarch's birthday is held on a Saturday in June when the weather is better. The custom dates from 1748 and will be June 11 this year.

William, riding a mottled gray horse called Wellesley, was wearing the red Irish Guards tunic with a blue sash that he wore at his April 29 wedding to Kate Middleton at Westminster Abbey.

William, 28, was made Colonel of the Irish Guards in February. He is expected to wear the same outfit at the actual "Trooping the Colour" ceremony, which usually draws thousands of well-wishers to the ceremonial heart of London.

The event Saturday, officially known as the Colonel's Review, is the second of two rehearsals for the queen's birthday festivities.

William and his new wife, formally known as the Duchess of Cambridge, plan to visit Canada and California in late June and early July.

They will visit several cities in Canada from June 30 to July 8, traveling throughout much of Canada on their first official overseas trip as a married couple.

They also plan a stop in Los Angeles and the surrounding area from July 8-10.