Several UK Muslim clerics have condemned the Islamic State militant group in Syria and Iraq (former ISIS) saying that British jihadis are “betraying their own societies” by getting involved in the Middle East conflicts.

The fatwa, an edict that concerns Islamic life, called the Islamic State (IS) “heretical” and “an oppressive and tyrannical group”, in the strongest denunciation against the extremists yet. The edict was issued by Sheik Usama Hasan, a former imam from east London, and got support from six senior Islamic scholars.

The fatwa states that although Muslims have a “moral obligation” to help the Iraqi and Syrian people, they must do so “without betraying their own societies”. So, it is “religiously prohibited to support or join” the IS, the document cited by The Sunday Times reads.

The edict comes as Britain has raised its terror threat level from “substantial” to “severe” in response to the risk of Jihadists returning to the UK from the conflicts in Iraq and Syria.

Theresa May the Home Secretary said that although an attack on the UK was “highly likely” there was no evidence that an attack was “imminent”.

UK jihadists may be banned from coming home

UK Prime Minster David Cameron is considering introducing a temporary ban on allowing UK born jihadists back into the country who have been to Iraq and Syria. Cameron will set out the plans in parliament on Monday, British media report.

UK nationals would be allowed to keep their nationality but would be barred from reentering the UK for a period of time. The Prime Minister is also expected to announce plans to make it easier for the authorities to seize the passports of suspected terrorists to stop them travelling abroad in the first place.

"The government is considering a range of measures to keep the country safe in the face of an increased threat level from Islamist extremism. The areas include making it harder for potential foreign fighters to travel abroad by making it easier to remove their passports through additional temporary seizure powers at the border,” a government source told the BBC.

"We are also looking at stopping British citizens from re-entering the country if they are suspected of terrorist activity abroad,” the source added.

But Sir Menzies Campbell, the former leader of the Liberal Democrats, the junior party in the current coalition government, said that the proposals by the Prime Minister might be illegal under international law and any ban introduced by the government would have to be tested in the courts.

"I think it's rather difficult and it might well constitute illegality. To render citizens stateless is regarded as illegal in international law. To render them stateless temporarily, which seems to me the purpose of what's being proposed, can also I think be described as illegal,” he said.

While former Royal Marine and intelligence officer Paddy Ashdown who was also leader of the Liberal Democrats told the Observer that the reaction of Conservative ministers to the terrorist threat was “kneejerk”.

He told the Observer that the biggest threat to the world was “a widening religious war, which threatens not just to engulf the Middle East and change its borders, but to spread across the entire global Islamic community".