Ask a group of schoolchildren what they know about treason and they are most likely to mention Guy Fawkes and Ann Boleyn - but a closer look at the law brings some more recent names to mind.

The first statutory definition of treason was set out in 1351, where alongside the better-known points about not planning the death of the King or helping his enemies, Parliament warned citizens not to “violate the King’s companion, or the King’s eldest daughter unmarried, or the wife of the King’s son and heir.”

So although new definitions of treason have been added over the years - and the punishment softened - the original statute has never repealed, which means violating the monarch’s eldest son’s wife is still an offence.

It won’t get you hung drawn and quartered like it would have done in the Middle Ages, or even just hanged like it did Lord Haw-Haw in 1946 – but anyone caught having sexual relations with the heir to the throne’s wife could end up with a lifetime behind bars pondering whether it was worth it.

But life imprisonment for treason is a modern idea. Until 1998 the penalty was to be put to death, and it was during a House of Lords debate on the issue that the Earl of Onslow mentioned what everyone else was thinking.

If treason was a capital offence, he told the Lords, then the Princess of Wales’ lover Major James Hewitt – who was, according to the Earl, “undoubtedly guilty” - ought to be sentenced to death.

This he said, was “ridiculous”, and we can only imagine what he would have made of last year’s inquest into Princess Diana’s death, during which the High Court was told that Major Hewitt was one of five men to have had sexual relations with Princess Diana while she was still married to Prince Charles.

Since no one has been charged with treason since 1946, when William Joyce, the Nazi propagandist better known as Lord Haw-Haw, was convicted and sent to the gallows, the prospects of a modern day prosecution seem slim.

But until Parliament acts on Lord Goldsmith’s suggestions there remains an outside chance we might see some interesting names in the fray for a treason trial.

Explore Law

• Columnists

• Law Panel

• Law Reports

• Student Law

Times Recommends

• Linklaters trounces rivals as UK's biggest firm
• Clifford Chance profits plunge 37 per cent
• Casual drinkers hit by spread of 'dry' zones