Where other young people may have expressed surprise at the theme, it would have barely caused a raised eyebrow for Prince William and Prince Harry, whose father has long been friends with Mr Meade.

Some of the party guests had taken part in the protest inside last year’s Labour conference against a ban on foxhunting.

William and Harry are linchpins of a set of young people linked to the Beaufort Hunt and the Beaufort Polo Club. Mr Meade’s children James, 23, Harry, 22, and Lucy, 21, are part of the set. They went to the same public schools; they partied with the Princes in village pubs around the Highgrove estate.

Last week the two young Princes and their friend Guy Pelly, 22, set off for Nailsworth, a few miles from Highgrove to choose fancy-dress outfits. They went to Cotswold Costumes, which proudly boasts more than 2,000, ranging in price from £15 to £20. William settled on a black leotard with a leopard-skin tail, leopard-skin paws and a leopard-skin pattern sewn into the coat.

Mr Pelly decided to dress as the Queen and chose a powder-blue dress, grey wig, long white gloves and a crown.

Harry went for the uniform of a Nazi officer complete with jacket and red swastika armband. The uniform cost £25 to hire with a £20 deposit to be repaid on its return.

Neither William nor Mr Pelly, the son of a Kent landowner, tried to dissuade Harry from his choice of “fancy” dress.

The Prince of Wales, who has engaged in only light official duties since the Christmas break, was at Birkhall, Queen Elizabeth the Queen Mother’s home on the Balmoral estate that he inherited on her death. Prince Charles had gone there after the Royal Family’s Christmas break at Sandringham. He interrupted his Highland stay briefly to visit charities involved in the tsunami appeal.

On Saturday evening the two young men, joined by Mr Pelly, who regularly accompanies them to England rugby matches, changed into their outfits at Highgrove while the Prince of Wales relaxed in Scotland with Camilla Parker Bowles.

No authority figure was present to remonstrate with Harry about his disastrously ill-judged choice of costume.

Only the ever-present royal protection officers were there. It is not part of their job description to issue sartorial advice to the third in line to the Throne.

If, however, any advice was proffered by the armed bodyguards, Harry ignored it and arrived with his brother and Mr Pelling at a party attended by 400 people. Many of the guests were strangers carrying mobile telephones; some had disposable cameras.

The marquee in the equestrian centre, as befits the theme, was decorated in white at one end and black the other. Mr Meade, who won three Olympic golds for three-day eventing, spotted Harry’s costume when he arrived but said nothing. It was a family party to mark Mr Meade’s recent 66th birthday and his son Harry’s 22nd.

Mark Tomlinson, who was at Eton with Prince Harry, was at the party. Harry stayed at the Tomlinsons’ ranch in Argentina during his gap-year trip.

Emma Wade, a photographer whose father, Captain Ian Farquhar, is Master of the Beaufort Hunt with which Prince William rides, was also present.

Mrs Wade was one of the protesters who disrupted Tony Blair’s speech in Brighton in October, as was another of the guests, James Murray Wells. Mr Murray Wells celebrated becoming a millionaire this week, having set up an internet website selling cut-price spectacles.

Harry, as usual, drank vodka and cranberry juice and chain-smoked. Mr Pelly brought the house down with a speech in which he mimicked the Queen’s voice. A pretty brunette sat on Harry’s lap. William also made a speech. They partied until 5am.

Harry did not know many of those present yet naively thought he was safe, being among friends from the hunting, shooting and fishing set.

It was only at 4pm on Wednesday that Clarence House had an inkling something was wrong. A telephone call came through from The Sun to Paddy Harverson, combative press secretary to the Prince of Wales. The newspaper told Mr Harverson that it had photographs of Harry in Nazi uniform, a red swastika on show. Mr Harverson immediately telephoned Harry and alerted the Prince of Wales in Scotland.

Harry swiftly admitted that he had made a mistake and said that Clarence House could issue a statement on his behalf expressing his regret. Yet he apologised “if I have caused any offence” — when clearly he had.

Mr Harverson, confident that the story would blow over, went to Stamford Bridge to watch Chelsea play his former employers, Manchester United, in the Carling Cup. It was a satisfactory result for Manchester United, who left London with a draw.

But the party affair did not end with the result that Mr Harverson wanted or expected, as leading figures across the globe joined in the criticism of the young Prince.

Yesterday morning Clarence House was battening down the hatches.

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