The foreign secretary has said ministers are talking about whether to prosecute Britons who fight for the Islamic State group in Syria or Iraq for the crime of treason.

Answering a question from Conservative backbencher Philip Hollobone in the House of Commons, Philip Hammond said: “There are a number of offences under English law with which returning foreign fighters can be charged.

“We have had a discussion about the allegiance question. We have seen people declaring that they have sworn personal allegiance to the so-called Islamic State.

“That does raise questions about their loyalty and allegiance to this country and about whether, as my hon. Friend rightly says, the offence of treason could have been committed.”

Edward III giving the Magna Carta, enlargement.

What is treason?

The law on treason is one of the oldest on the statute books. Much of it dates back to the treason act of 1351, passed in the reign of Edward III at a time of almost constant war.

Some of the act – still available to read here in English and in the beautiful original Norman French – is still good law, although much of it has been repealed or amended in later legislation.

As far as Brits who go to fight for Islamic State group are concerned, the relevant section of the 1351 act is about those who “levy War against our Lord the King in his Realm, or be adherent to the King’s Enemies in his Realm, giving to them Aid and Comfort in the Realm, or elsewhere”.

The punctuation at the end of that last phrase makes it unclear whether the law was supposed to cover treasonous acts in the King’s realm alone or abroad – “elsewhere” – too.

The controversial trial of the Irish nationalist Roger Casement in 1916 settled that point. Although Casement’s crimes were carried out in Germany, it was decided that the unpunctuated Norman French version of the treason act covered all offences “in the Realm or elsewhere”. Some said Casement was “hanged on a comma”.

Treason only applies to people “owing allegiance” to the Crown – British citizens who betray their own country.

Though ancient, the act was used as recently as 1946 to execute a (supposed) British citizen who acted on behalf of the king’s enemies during wartime.

Arguments about whether treason is still enforceable tends to focus on whether it is needed in a more peaceful age, whether attitudes about loyalty to queen and country have changed and whether a modern prosecution could work when the law is so rarely used.

The original law made a distinction between “high treason” – an offence committed against the sovereign – and “petty treason” – where the victim was a lesser mortal. But petty treason was abolished in 1828 so the word “high” is redundant now.

We often think of high treason as an attempt on the monarch’s life, as in Guy Fawkes’s Gunpowder Plot of 1605, but other offences can also be considered treasonous.

Princess Diana Retrospective

The act also says it is an offence “if a Man do violate the King’s Companion, or the King’s eldest Daughter unmarried, or the Wife of the King’s eldest Son and Heir”.

There was idle speculation that alleged lovers of the late Diana, Princess of Wales, who slept with her while she was still married to Prince Charles, could technically have been prosecuted for treason – which, at the time, still carried the death penalty.

Can you be hanged for treason?

No. It’s a persistent urban myth that a number of obscure offences on the statute book are still punishable by death. They aren’t.

The confusion might come from the fact that, while capital punishment for murder ended in 1965, a few rarely-prosecuted crimes still carried the death penalty for decades afterwards.

Treason was one of them. The others were espionage (under military law), piracy with violence and arson in a naval dockyard.

Death sentences for the arson and spying offences were abolished in 1971 and 1981 respectively. The death penalty was abolished altogether in British civil law by the 1998 Crime and Disorder Act.

A 2004 amendment to the the human rights act outlaws the use of the death penalty “in all circumstances”.

Public Execution

Methods of execution for treason tended to be the most gruesome available. People found guilty of high treason were routinely hanged, drawn and quartered or burnt at the stake, their bodies sometimes being displayed in public.

The practice of quartering – dividing the body into four pieces – was only abolished in 1870. And beheading was only abolished as an alternative punishment in 1973. In fairness, both practices had long been obsolete by the time they were officially outlawed.

When were treason laws last used?

The Nazi sympathiser William Joyce, known to British radio listeners as Lord Haw-Haw after making pro-German propaganda broadcasts with an affected upper-class accent, was the last man to be hanged for treason.

Joyce was executed in 1946 after a trial that has been widely criticised by legal commentators.

Lord Haw Haw

The case for treason rested on him being a British passport-holder at the time he made the broadcasts on behalf of Nazi Germany.

It turned out that Joyce had been born in America and had obtained a British passport by falsely claiming to be a British subject. But possession of the passport was enough to prove he was “a person owing allegiance to our Lord the King” and send him to the gallows.

Joyce is not the last person to be convicted under treason laws.

In 1966 17-year-old John Francis Morgan was jailed under the treason act 1842, which created a new offence of “throwing or using any offensive matter or weapon with intent to injure or alarm” the monarch, after he dropped a concrete block on the Queen’s Rolls-Royce as she rode through Belfast in 1966.

In 1981 another teenager, Marcus Serjeant, was given five years after he fired six blank shots at the Queen as she rode down the Mall in central London to the trooping the colour ceremony. He pleaded guilty an offence under section two of the 1842 act.

MI5 officer Michael Bettaney is sometimes wrongly said to have been jailed for treason in 1984 after spying for the Soviet Union. Bettaney was in fact convicted of offences under the Official Secrets Act.

Is defacing a banknote treason?

Again, another legal myth is that defacing the queen’s head on stamps or banknotes is technically treason. This is nonsense, according to the Law Commission.

The currency and banknotes act 1928 and the coinage act 1971 do make it an offence to deface notes and coins respectively, including by stamping on notes, but this is does not add up to treason.


Could treason be used today?

Legal experts have cast doubt on whether the most serious offences first set out in 1351 could be prosecuted now.

Cases come to court so rarely that you would struggle to find a judge or barrister with any knowledge of the law.

In correspondence from 2008, the Ministry of Justice’s criminal law and legal policy unit confirmed that “treason is still regarded as a crime in the UK” but said: “Many of the acts are couched in archaic language, and prosecutions are liable to be rendered difficult and complicated by the necessity to prove all the ingredients of the offences.

“In many circumstances alternative offences are available and might be preferred as being more straightforward in prosecuting the case.”

Terrorism legislation covers most of the crimes jihadis could commit against the UK while fighting in Syria or Iraq, and previous governments have declined to use treason against Britons who join foreign insurgents when faced with similar calls from MPs.

A 2008 report by the Law Commission said: “There is a need to consider the justification for the existing treason offences. In the Middle Ages, England was more often than not at war. It is questionable whether treason offences are required in peacetime.

“In the eighteenth century there were serious rebellions bordering on civil war. Nowadays, we have offences such as riot to cope with serious civil unrest. Terrorism is a modern phenomenon and the treason offences are ill-equipped to deal with it. Instead, we now have discrete terrorism offences.

“Treason is an area where the criminal law could be simplified and pruned. It is an example of an area of the law shaped by political and social conditions that have ceased to be of contemporary relevance.”