The goverment is considering invoking the charge of treason for outspoken Islamist clerics.
William Joyce was the last man hanged for treason in the UK
It is unlikely to happen, according to BBC legal affairs analyst Jon Silverman.
BBC News website's Katherine O'Shea looks at the handful of cases in which it has been used in the past century.
1916 ROGER CASEMENT
The renowned human rights campaigner
was convicted of high treason for his involvement in the Irish Nationalist revolt in Dublin.
He was hanged at Pentonville Prison.
Executioner Albert Ellis recalled: "He appeared to me the bravest man it fell to my unhappy lot to execute."
1945 JOHN AMERY
As a British anti-Communist, John Amery made recruitment attempts and propaganda broadcasts for Nazi Germany.
His father was a cabinet minister, while his younger brother became an MP and was elevated to the peerage.
Amery was hanged for treason after the war.
Initially he pleaded not guilty, while his counsel attempted to show that he was mentally ill. This strategy failed and he pleaded guilty at his trial to eight charges of treason.
This is the only known case of a man pleading guilty to a charge of treason in the UK.
1946 NORMAN BAILLIE-STEWART
Known as 'The Officer in the Tower', Baillie-Stewart was the last British citizen to be imprisoned in the Tower of London.
He spent five years there after being convicted in 1933 under the Official Secrets Act of selling information to the Germans.
Baillie-Stewart was later charged with treason for taking German citizenship during World War II, but the prosecution was forced to drop this charge due to a technicality.
He pleaded guilty to committing an act likely to assist the enemy and was sentenced to another five years' imprisonment in 1946.
1946 WILLIAM JOYCE
Known as Lord Haw Haw because of his nasal drawl, Joyce was a fascist politician and Nazi propaganda broadcaster to the UK during World War II.
He defected to Germany, but at the end of the war was captured by British forces near the German-Danish border.
Joyce was convicted on three counts of high treason, and executed at Wandsworth Prison.
He became the last man hanged for treason in the UK
1981 MARCUS SARJEANT
Seventeen year old Sarjeant fired blank shots at the Queen during a Trooping the Colour ceremony. The Queen was unharmed.
Sarjeant pleaded guilty to a charge under the 1848 Treason Act and was jailed for five years.
He wrote to the Queen from prison to apologise for firing the blanks, but he never received a reply.
Sarjeant was released in October 1984, at the age of 20. He changed his name and began a new life.
1984 MICHAEL BETTANEY
An MI5 officer, Bettany was convicted at the Old Bailey of treason after spying for the Russians and jailed for 23 years.
He was released on parole in 1998.