British jihadists who go to fight in Iraq or Syria could be tried for treason, the Foreign Secretary has suggested.
Philip Hammond said such people had "sworn personal allegiance" to Islamic State (IS) and could potentially have committed the offence.
The last prosecution for treason was in 1946, when the notorious Lord Haw Haw - real name William Joyce - was hanged for his Nazi propaganda broadcasts.
The offence remains on the statute books, even though the death penalty has been abolished.
Questioning Mr Hammond in the Commons, Tory backbencher Philip Hollobone said: "Given that, with the assent of this House, Her Majesty's armed forces are now engaged in military action against Islamic State, given that we have all witnessed on television the beheading by a British jihadist of British and American aid workers, and given that the offence of treason still exists, but has not been used since 1946, will the Foreign Secretary ensure that British jihadists who return from Iraq and Syria are prosecuted for the offence of treason?
"Their 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."
Mr Hammond replied that there were "a number of offences under English law with which returning foreign fighters can be charged".
He said: "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 honourable friend rightly says, the offence of treason could have been committed.
"I will certainly draw his remarks to the attention of the Home Secretary, who ultimately will be the person who needs to look at this."