Exams have to come first for Prince William (AP)

Prince William, Britain's future king and a heartthrob for girls around the world, was a notable absentee from the huge party that the royal family threw at Windsor Castle near London that likewise commemorated five millennium milestones and 260 years of royalty.

While Prince William buried his nose in his books at Eton College, the Queen Mother, who is 100 on 4 August was the toast of the evening.  European and Scandinavian royalty, from Spain, Belgium and Norway, were present, including ex-King Constantine of Greece, who now lives in Hampstead, north London.

The dinner guests were joined later by about 700 others for a drinks party in the castle's magnificently-restored Saint George's Hall and adjoining rooms.

The daily tabloids however attributed Prince William's absence on an informally reported rift between Prince William and Charles, because Charles wanted to invite to the party his partner, Camilla Parker Bowles.  Camilla was not present in the important royal family occasion.

Prince William is writing his A level examinations at the elite Eton College close to Windsor Castle.  He is due to sit an exam on the history of art Thursday.  He is also Eton's swimming champion and all-time record holder and captain of Eton's football team. Prince William is expected to go on to Edinburgh University.

The broadcasting stations paid their own tribute to Prince William's birthday by playing "God Save The King" before the early morning news - an honour given only to the most important royals.

Prince William does go online often (AP)

 The prince, who has postponed using the HRH title in favour of just William, has said he feels uncomfortable with the public and media attention focused on him but recognizes it is something he will have to get used to.

Prince William, whose mother Princess Diana, was hounded by prying lenses wherever she went, pleaded for the paparazzi to leave him alone at the university and refused to say if he had a girlfriend.  "I like to keep my private life private," he said in a written interview.

The prince, who like Princess Diana has stunning good looks, is clearly uncomfortable with the voracious tabloid appetite for endless stories on the young bloods in the royal House of Windsor.  But he was grateful that the media had left him and his 15-year-old brother Prince Harry alone during their Eton school years.

The media, vilified after Princess Diana's 1997 death for constantly pursuing her, agreed under an informal arrangement with the industry's watchdog that school pictures were strictly off-limits.