Statisically, the broadcast was a huge success: 18 million people tuned in, the fourth biggest audience for 1987, and more than 400 million viewers worldwide saw it later.
But for the dignity of the British monarchy, It's A Royal Knockout was a disaster. Prince Edward, Princess Anne, Prince Andrew, and most of all Andrew's wife, Sarah, lent themselves to a custard-pie TV game show that had become a byword for inanity.
It was all good clean fun; it was for charity (it raised pounds 1.5m). But by donning Olde England fancy dress and cavorting through a series of party games with a bunch of "celebs" dressed as squires, damsels and minstrels before a "medieval castle" knocked up at Alton Towers amusement park, the young royals inadvertently mocked the real costumes and ceremonies of their own House of Windsor. It was the breaking of royalty's magic spell.
More than that, it was the harbinger, many commentators feel, of all the royal troubles that were to come. And if future historians conclude that the divorce last Wednesday of Andrew, Duke of York, from his Duchess marks the end of something, they may well feel that 17 June 1987 marked its beginning.
That something was the irruption of showbiz into the royal world, and all the damage that caused.
A senior BBC executive involved with It's A Royal Knockout last week likened the occasion and change in public attitudes to the night when the former newsreader Angela Rippon revealed her thighs (through yards of chiffon) while dancing on The Morecambe and Wise Christmas Show in 1976.
"Suddenly she was showbusiness," he said. "There is no question that the following morning everything had changed. No longer was Ms Rippon a serious journalist to a wide section of her colleagues and the public. She had entered the showbusiness arena as a personality and had taken the industry in a different direction.
"Some established older newsreaders were appalled. And on that day in 1987, the royals put themselves on the line as well. Prince Edward might have been the instigator, but the Duchess of York got them going.
"Prince Andrew was never going to make it in a double-act with her. She needed another kind of stronger personality who would lead her, and that showed even then."
Prince Edward was indeed the moving spirit behind the royal revival of a buffoonery session which had flourished on TV from 1966 to 1982. Yet both the Queen and Prince Philip were dismayed by Edward's unconventional wheeze to raise money for four charities: The Duke of Edinburgh's International Project 1987, the Save the Children Fund, the (then) World Wildlife Fund and the International Year of Shelter for the Homeless.
Prince Philip felt their children's participation was "unwise and unwelcome". He told theBBC executive quoted above: "Why doesn't Edward let the TV people get on with it and just turn up to accept the cheques? He's making us look foolish."
The stage-struck youngest son paid no heed. He approached Stuart Hall, the jovial Mancunian presenter of the programme until its demise, and in the grounds of Buckingham Palace, says Mr Hall, they wrote down the formula for the royal special on the back of an envelope. It involved 40 celebrities, including Cliff Richard, Christopher Reeve, John Travolta, Jane Seymour, Anthony Andrews, Chris de Burgh and Meatloaf, divided into four teams for each of the four charities; each team had a royal captain.
The Prince laboured on his first TV production for six months, but the day did not go well. The reptiles of Fleet Street were excluded from the proceedings and forced to watch the show on a monitor screen in a tent. When Edward (whose sweatshirt, emblazoned with the slogan, No. 1 - Just Look Like Him, was unfavourably commented upon) asked if the journalists had enjoyed themselves, they failed to respond altogether positively.
"Thanks for sounding so bloody enthusiastic!" he shouted, before storming out of the press conference - which of course captured the next day's headlines, which were hostile in the extreme. Stuart Hall, however, won't hear a word said against Edward or the other royals. "I thought it was a marvellous effort by them all to turn up and perform on the day for a big jolly and enjoy themselves," he said last week. "Prince Edward is one of the brightest guys I have ever met. They may have looked foolish, but that was the fun of it.
"Tom Jones was on a ladder he couldn't quite climb up ... Jane Seymour gave him a poke up the bum ... Tessa Sanderson threw a javelin at John Travolta ... who was giving a big kiss to the cricketer Viv Richards. They had a joust with the Royal Family and raised a lot of dosh. It should have been an annual event.
"Fergie had a whale of time. It is churlish of the British public now and typical of the British press today that they attack everybody. Everything is so negative here. This is the greyest country in the world. We are a joke. I have just come back from the Middle East. They can't believe what is happening in this country. The press attacks everything in sight. No praise is given where praise is due.
"Fergie is a boisterous red-headed girl out for fun and everyone just attacks her. I feel sorry for her, I really do. There is so much criticism of her. I am a royal supporter. Having worked with them, I have great respect for them, especially for Anne and Edward. What do you want, the Royal Family to be King George and Queen Mary, never seen and never heard, waving a hand occasionally? We are a giant theme park and they are the main attraction. We didn't bring them into disrepute. The public is wiser than that."
Many people disagree. Philip Ziegler, the distinguished royal biographer, said: "It would surely have been possible for [the Queen] to take a stronger line over such an ill-judged enterprise as the It's A Royal Knockout on television? Perhaps the full horror of this extravaganza did not become apparent until it was too late; if so, the Queen was remarkably ill-informed. Someone should have warned her what was about to happen so that she could have taken steps to stop it ,or at least moderate its excesses."
James Whitaker, royal correspondent of the Daily Mirror, maintains that this one programme was a "total watershed" in the public's perception of the Royal Family. "If you really had to pin down where it all went wrong, I would always point to It's a Royal Knockout," he said. "That was the start of the high-profile thing that started everyone thinking, `Who are these appalling people?'"
It wasn't only the House of Windsor that was damaged, either. Mr Hall still bears the scars of the day. Prince Andrew accidentally fired a cannon prematurely, knocking the presenter off his feet and singeing his eyebrows. "Another two feet closer to the camera and he would have blown my bloody head off," he said last week. "I was sent flying. But that was part of the day."
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