It would not be the first time a member of the Royal Family has been a witness in court proceedings, but their presence always raises the news value.
Queen Victoria's eldest son, later Edward VII, gave evidence in a slander action brought by Sir William Gordon Cumming, accused of cheating at cards by a shipping millionaire, Arthur Wilson. Despite the Royal intervention, Sir William lost his case, had to resign his commission in the Scots Guards, and was ruined.
Newspapers carried long accounts of all seven days of the trial. Queen Victoria demanded to be kept informed of developments, and a play based on the case was soon running in London.
In March 1990, Viscount Linley, who was 12th in line to the throne at the time, was awarded pounds 35,000 damages against the Sun's stablemate, Today, over a story saying he had been banned from the Ferret and Firkin pub in Chelsea for throwing beer over friends.
He had gone into the witness box to say how upset he had been that the article made him look like a 'hooray Henry', and that he had only visited the pub once, and with only one other person. But immediately after the case he agreed to waive pounds 30,000 of the damages in return for Today agreeing not to prolong the case by appealing.
Major Hewitt's solicitor, Mark Stephens of the London solicitors Stephens Innocent, said the writ referred to Tuesday's Sun. One article that day was headlined: 'Night Diana wept over Major'. Another was headed 'Di's Anger at Kiss and Tell Major'. Mr Stephens said the nature of the allegations meant his client 'felt it necessary to bring this action'.
Technically, either side can subpoena witnesses if they can be shown to be relevant, forcing them to give evidence.
Tom Crone, legal manager for the Sun, said yesterday that no writ or communication had yet been received from Major Hewitt or his solicitors, and until he saw the exact nature of the complaint, the paper could not say who would be called to give evidence for the defence.