UK Citizens Who Join ISIS Could Be Charged With Treason: British Foreign Secretary

British citizens who fight for the Islamic State group or swear allegiance to other militant organizations could be charged with treason, according to U.K. Foreign Secretary Philip Hammond.

Hammond revealed that there had been “discussion” within the U.K. government about whether British citizens who had sworn allegiance to a foreign state or militant group could be charged under the U.K.'s treason law, which dates back to 1361. Hammond said that jihadists from the U.K. who go to fight in Iraq or Syria could face trial under the antiquated law, in response to a question in the House of Commons from Conservative MP Philip Hollobone, according to the BBC.

Hammond added that because jihadis have "sworn personal allegiance to the so-called Islamic State" it raises questions about whether "offenses of treason have been committed."

Hollobone said: "[U.K. jihadists'] actions are treachery against Her Majesty, and aiding and abetting enemies of Her Majesty is one of the greatest offences a British citizen can commit."

However, lawyers cited in a report from The Times of London dismissed the proposal as “ludicrous.” Geoffrey Robertson, a leading human rights lawyer, told the paper that any attempt to prosecute a jihadist as a traitor would trigger protracted legal fights.

“This obsolete 1361 law... only remains on the statute books because of the laziness of law reform and our nostalgia for history. It is precisely that nostalgia which should deter us from treating vicious Islamic State killers on a par with Sir Roger Casement, Anne Boleyn and Sir Walter Raleigh.”

This is not the first move by the U.K. government to punish citizens who have joined the ranks of the Islamic State group, or ISIS.

Former shadow home secretary David Davis was one of a number of lawmakers who had called for Britons who joined ISIS to be stripped of their citizenship, according to a report from The Telegraph. Prime Minister David Cameron has also mooted a plan to strip jihadis of their passports.

Estimates of the number of U.K. citizens who have joined ISIS' ranks range from 500 to 1,500, according to U.S. and U.K. intelligence. "Jihadi John," the ISIS militant who has featured in a number of gruesome videos of Western hostages being beheaded, is believed by U.S. and U.K. intelligence agencies to be British.

There have been widespread reports in the U.K. media in recent weeks that a number of U.K. citizens who had joined ISIS had become disillusioned with the group and wanted to return to Britain, but feared prosecution if they did so.

Britain's treason law is legally archaic. The last person to be convicted under the statute was William Joyce, a propagandist for the Nazi regime, also known as Lord Haw-Haw. He was hanged in 1946. Although capital punishment has since been abolished in the U.K., the offense of treason still remains on the statute books.