Charles and Philip may be called as witnesses

The royal family have so far sidestepped the entire Diana inquest, now into its sixth month, but both the Prince of Wales and Duke of Edinburgh could soon find themselves subpoenaed to give evidence in the trial of two men who allegedly plotted to blackmail a member of the Royal Family.

Ian Strachan, 30, and Sean McQuigan, 40, are due to appear at the Old Bailey next month accused of demanding £50,000 from the unnamed royal, who for legal reasons cannot be named. Last August the pair threatened to release a mobile phone video in which one of the royal family’s assistants boasted of engaging in a homosexual sex act with the royal who is a married man. They were duly caught after the royal complained to the police, who set up a sting operation at a London hotel.

While Buckingham Palace has already indicated the alleged victim is not a member of the Queen's immediate close family, he is an heir to the throne. Prince Charles and the Duke of Edinburgh may now be drawn in to the controversy after Strachan told police that they could both back up his claim that he met the alleged victim. Representing Strachan is Giovanni di Stefano, a high-profile lawyer whose previous clients include executed Iraqi dictator Saddam Hussein.

The royal family member at the centre of the allegation has been named on US television and on the internet. Despite the news blackout in Britain, the alleged victim now fears his identity may be exposed in the same way that Prince Harry's secret army mission to Afghanistan was reported in Britain after being picked up by the American blogger Matt Drudge.

In his interview with detectives, the royal family member, asked how the affair had affected his family, said: "Excruciating pain and unbelievable amounts of anguish... it's been unbelievably stressful on people who I work with, on my friends, on my family.”

It is the first blackmail case involving a British royal in more than a century. In 1891, the then future king Edward VII discussed with his solicitor paying off two prostitutes he frequented in return for letters he had written to them.