Muslim leaders have issued a fatwa condemning British jihadis.
The fatwa, which The Sunday Times newspaper said had been issued by imams, is the strongest condemnation yet of Britons joining Islamic extremists by the Muslim community.
It comes as Britain's terror threat was hiked from "substantial" to "severe" in response to conflicts in Iraq and Syria, and the seemingly growing influence of the Islamic State terror group.
Home Secretary Theresa May said the new alert level rated the risk of an attack on the UK as "highly likely", although she said there was no evidence to suggest one was "imminent".
A fatwa, an edict issued by a learned Muslim scholar, may concern any aspect of Islamic life.
Technically it cannot be revoked and dies only with the person it is directed against.
Six senior Islamic scholars have endorsed the fatwa, describing Britons allied to Islamic State cells as "heretics".
According to the newspaper, the fatwa states Muslims have a "moral obligation" to help those in war-torn Syria and Iraq, but that they should do so "without betraying their own societies".
The term fatwa became famous in the West in 1989 after the author Salman Rushdie was forced into hiding following a ''death fatwa'' issued by Ayatollah Khomeni, then Supreme Leader of Iran, on the grounds that his book, The Satanic Verses, had ''insulted'' Islam.
The book's publication a year earlier triggered a wave of protest and condemnation from Muslims who accused it of blasphemy and mocking their faith.
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