REUTERS – Britain may use a medieval law dating to 1351 to charge citizens with treason if they go to fight with Islamic State insurgents in Iraq and Syria, according to Foreign Secretary Philip Hammond.



Security officials say some 500 Britons - almost all with Muslim immigrant backgrounds - are believed to be fighting in Iraq and Syria, though the true figure could be much greater. The concern is that those who return could carry out attacks on British targets.



Hammond said any British citizen who had sworn personal allegiance to the so-called Islamic State could have committed an offence under the Treason Act of 1351, which was passed during the reign of English King Edward III.



"We have seen people declaring that they have sworn personal allegiance to the so-called Islamic State," Hammond told parliament last week.



"That does raise questions about their loyalty and allegiance to this country and about whether the offence of treason could have been committed," he said, adding he would bring the issue to the attention of Home Secretary Theresa May.



Islamic State has released videos of the beheading of two American and two British men which feature a masked, black-clad militant brandishing a knife and speaking with an English accent who has been dubbed "Jihadi John" by British media.



The maximum sentence for treason in Britain is life imprisonment. Until 1998, the maximum sentence was death.



The last person to be hanged for treason in Britain was William Joyce, a propagandist for Nazi Germany. Nicknamed Lord Haw Haw, Joyce broadcast to Britain during World War II and was executed in 1946.



Britain has been considered a major target for Islamist militants since the 9/11 Al-Qaeda attacks on its close ally the United States. Fifty-two people were killed when four young Britons carried out suicide bombings in London in 2005.



Earlier on Friday, Mark Rowley, Britain’s national policing spokesman for counter-terrorism, said police were carrying out security investigations at an “exceptionally high” rate, with 218 arrests so far this year.



He said they were preventing several attack plots of “varied sophistication” a year, all inspired by terrorism seen overseas.