What do you get Britain's richest woman for her birthday? Another birthday, and another, and another.

While Queen Elizabeth celebrates her actual birthday on 21 April, she also has an official birthday, marked in the UK on a June Saturday, this year on 16 June.

But pity the poor chap at the Foreign Office who has to keep tabs on the Queen's various official birthdays around the world.

Official birthdays became separated from actual birthdays earlier this century when King Edward VII, born in November, began celebrating in the summer.

Out-of-date oddity

Harold Brooks-Baker, pubishing director at Burke's Peerage, thinks the whole idea of official birthdays is an out-of-date oddity which should have been tackled as part of Tony Blair's proposed "modernisation" of the monarchy.

"It's a nonsense," he said, "It's a legacy of the days when there were no faxes or telephones and celebrations could not be co-ordinated."

Mr Brooks-Baker said the separate birthdays confuse the public and are an example of Buckingham Palace's poor grasp of public relations.

For the first seven years of Queen Elizabeth's reign, her birthday was celebrated officially on the second Thursday of June.

But when - like another favourite date in the royal calendar, Derby Day on the Epsom Downs - midweek became inconvenient it was switched to Saturdays.

Trooping of the Colour

Elizabeth Alexandra Mary Windsor's arrival in the world - officially timed at 2.40am on April 21, 1926 - is toasted at parties in British embassies around the globe, usually at some time in June.

In Britain, the first Saturday in June is traditionally marked by the Trooping of the Colour ceremony in Horseguards' Parade.

For many years the Queen arrived at the party side-saddle on her favourite horse, Burmese, but when the old fella was put out to grass 10 years ago, she dismounted.

Now the 72-year-old monarch is driven past the guard of honour in a horse-drawn carriage.

Her official birthday is also marked by the flying of the Union flag on government buildings, a 42-gun salute and the publication of the birthday honours list.

Public holiday

Elsewhere Australians and New Zealanders mark the occasion in style by taking the day off work. Canadians mark the event on May 18.

Most of Australia marks the official birthday as 8 June but Western Australia decided long ago to move the date to September 28.

In New Zealand, the first Monday in June is the Queen's official birthday.

But a spokesman at the New Zealand High Commission in London said: "It doesn't amount to much. It's just a public holiday."