British extremists who have travelled to Iraq and Syria and pledged their allegiance to Isis could potentially be charged with “high treason”, the foreign secretary, Philip Hammond, told MPs in parliament. But what exactly is the history of this 663-year-old act?
At the core was the politics of betrayal. It was first codified in English law as the 1351 Treason Act during the reign of King Edward III, distinguishing between high treason, which was against the crown, or petty treason, the disloyalty to a subject. It clearly states that one is guilty when:
A man doth compass or imagine the death of our lord the King, or of our lady his Queen or of their eldest son and heir.”
“Violating” your majesty’s wife, the sovereign’s eldest daughter, or the wife of the heir to the throne were also treasonous acts. If you waged war against the king, aided the enemy or even killed the king’s chancellor, your crime was punishable by death.
This crime had one of the goriest punishments in English history.

First, the traitor would be tied up and drawn across rough ground by a horse. He would then be hanged to within an inch of his life. He would be disembowelled and beheaded. Then he would be cut into four sections.
The public would watch and sometimes throw rotten food. If the traitor was a woman, she was burned. The bloody details were shared in penny pamphlets and engravings depicting the slow execution.
One of the most famous traitors in English history was Guy Fawkes, a Catholic Yorkshireman who in 1605 was arrested while guarding 36 barrels of explosives beneath the House of Lords. He was said to be planning the assassination of King James I with others in the Gunpowder Plot. He had been interrogated several times but admitted to almost nothing.
The Crown and Church were still inextricably linked in the early 17th century and this plot was seen as part of the broader Papal threat across Europe. To this day, in the UK people still commemorate Guy Fawkes Night with bonfires and fireworks on 5 November, even if the strong anti-Catholic sentiments no longer remain.

The Treason Act is one of the earliest statutes still in force today – albeit with significant amendments.
The last time it was used in the UK was in 1945, to prosecute William Joyce, or Lord Haw-Haw, a Nazi propagandist who had aided and assisted Germany during the second world war by:
Broadcasting to the subjects of our Lord the King propaganda on behalf of the said enemies of our Lord the King.”
However, since the 1998 Crime and Disorder Act became law, the maximum sentence for treason in the UK is life imprisonment.