Two men are beginning five-year jail sentences today after being found guilty of trying to blackmail a member of the Royal Family over claims that he took drugs and performed a gay sex act on his aide.

Ian Strachan, 31, and Sean McGuigan, 41, demanded £50,000 from the royal after secretly filming a member of his staff making lurid claims about his boss and “slagging off” other members of the Royal Household.

In the first public case of blackmail involving a member of the Royal Family since 1891, the Old Bailey was told that the men had made eight hours of tapes of the aide making bizarre and often ludicrous claims.

Filmed snorting cocaine, the aide — likened in court to John Inman’s character in Are You Being Served, such was his campness — said that the royal had performed a sex act on him on the kitchen floor at a party as a stripper looked on. However, the royal in question denied the claims vehemently and defence barristers admitted that they were untrue.

The tapes also showed the aide being rude about members of the Royal Household and their friends.

Last year Strachan and McGuigan hawked the tapes around tabloid newspapers in an attempt to make hundreds of thousands of pounds. When newspapers refused to publish the “scandalous” claims, the men turned to the royal himself.

The royal’s staff said that “menacing phone calls” — some of which made little sense — were accompanied by laughter in the background.

Strachan, who had a drug habit, forged a contract with the News of the World in an attempt to prove that he was on the verge of publishing the lurid accusations on the tapes.

The pair were eventually caught during an elaborate sting at the Hilton Hotel last September, when they showed the tapes to undercover officers posing as representatives of the royal.

During the three-week trial the court was at pains to protect the identity of the royal, named only as Witness A, and his aide, Witness D. Neither man gave evidence in court, which occasionally excluded the media and public for the most sensitive evidence.

The identities of 30 witnesses were also kept secret from the public.

The case focused on the extraordinary antics of the defendants; Strachan, a Walter Mitty fantasist and self-proclaimed socialite, who boasted of being on the fringes of the royal society scene, and McGuigan, a recovering alcoholic.

In the witness box, Strachan was challenged repeatedly about his lies. He had posed as a solicitor, despite dropping out of a law access course in Edinburgh after only a few months.

Strachan went to lengths to suggest that he mixed in royal circles, but it became evident that the closest he had reached to that society was meeting the aide. It was after that meeting that he hatched the blackmail plot and secretly recorded him.

Sentencing Strachan, from Fulham, and McGuigan, from Battersea, Mr Justice Cooke said: “This offence has been described as one of the ugliest and most vicious crimes in the calendar of criminal offences, and was described in this court as a dirty, filthy and hideous crime.

“It puts the victim in a position of fear. It puts the victim into a state of anxiety and torment.”

He condemned Strachan for being motivated by “greed and revenge” while McGuigan told “lie after lie” to try to secure the £50,000 payment.

The blackmailers showed little emotion as the guilty verdicts were read out. Strachan, wearing a smart dark suit, looked up and let out a heavy sigh as he was found guilty.

McGuigan has previous convictions dating back 22 years. One conviction was for deception and another for serious assault in 1999, for which he served three years in prison. Other offences included common assault, drugs offences and motoring matters.

“The corrosive effect of blackmail means that any sentence in a case of this kind must have a deterrent effect,” the judge told Strachan and McGuigan. They will have to serve half their terms before parole is possible.

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