But as he headed for the royal party's tea tent he was rugby tackled by a Beefeater in red and gold uniform to cheers from guests.

The Duke of Edinburgh, Prince Charles, Princess Anne, Prince Edward and wife Sophie Wessex were also at the event. Defence Secretary Geoff Hoon was said to be there.

A witness who gave his name as Mark said: "He was around 10 metres in front of the Queen. His blue and white striped boxer shorts were around his ankles. We saw everything."

Becky Fisk, 25, of Chislehurst, Kent, added: "He was shouting 'Wahey'. " Her dad Victor said: "He started in the middle of the lawn where the Queen was coming up. The Beefeater ran after him and rugby tackled him. They whisked him away."

The incident comes a month after "comedy terrorist" Aaron Barschak, 36, gatecrashed Prince William's 21st birthday party at Windsor Castle.

Despite the strict dress code the teenager got into the Palace without morning dress or suit.

But Scotland Yard insisted he was not an intruder, saying: "He was an invited guest. He was questioned and released but not arrested. There was no breach of security or risk to the royals."

The teenager, who was at the party with his father, a teacher, was today said to be "extremely embarrassed. Neither the boy nor his father have been identified, but in a statement the father said: "I would like to apologise for the ill-considered actions of my son. He is extremely embarrassed and regrets any discourtesy."

A spokesman for Buckingham Palace said he understood the teenager was writing a letter of apology to the Queen and added: "We regard the matter as closed."

-THE song Nowhere Fast on The Smiths' 1985 album Meat is Murder includes the lines: "I'd like to drop my trousers to the Queen, Every sensible child will know what this means."

Do you know the palace party prankster? Please ring the Daily Mirror newsdesk on 0207 293 3831.