A group of leading British Muslim clerics issued a fatwa outlawing the Islamic State group and prohibiting local UK Muslims from fighting on behalf of any side in the Syrian civil war, The Sunday Times reported.

Calling the IS doctrine “poisonous ideology,” the missive, endorsed by six leading Islamic leaders from across the UK, called on British Muslims to oppose IS, and in particular within the UK.

“There is no doubt that President Assad’s regime in Syria is oppressive, unjust and brutal, and has committed numerous atrocities against its own people,” the fatwa read. “The same is true of the so-called ‘Islamic State’ (IS) or self-styled ‘Caliphate, formerly known as ‘The Islamic State in Iraq and Syria': it is an oppressive and tyrannical group.”

The move came after mounting criticism charging that Britain’s Muslim leaders were not doing enough to counter the trend of UK Islamists traveling to Syria to join IS and fight on its behalf, the report said.

“The IS persecution and massacres of Shia Muslims, Christians and Yazidis is abhorrent and opposed to Islamic teachings and the Islamic tolerance displayed by great empires such as the Mughals and Ottomans,” continued the fatwa. ” IS is a heretical, extremist organisation and it is religiously prohibited (haram) to support or join it; furthermore, it is an obligation on British Muslims to actively oppose its poisonous ideology, especially when this is promoted within Britain.”

The religious deceleration also instructed Muslims to live by the law of the land in which they reside.

“British and other EU citizens are bound by their duties to their home countries according to Islamic theology and jurisprudence: it is therefore prohibited (haram) to travel to fight with any side in Syria, including non-state actors, since this is forbidden by laws in EU countries.”

Britain recently raised its terror threat level from substantial to severe amid fears of attacks by returning jihadists.

Security officials quoted in the report said Prime Minister David Cameron was set to announce measures aimed at preventing British jihadists who fought in Syria or Iraq from returning to the UK by canceling the passports of those suspected of involvement in terror activity.

The imams, from Manchester, Leeds, Birmingham, Leicester and London, included Sheik Qamaruzzaman Azmi, secretary general of the World Islamic Mission; and Sheik Muhammad Shahid Raza, executive secretary of the UK Muslim Law Shariah Council and head imam at Leicester Central Mosque.

“Many scholars from diverse theological backgrounds and schools are supporting it so it carries a lot of weight,” Raza said. “I hope that our young people will listen to what we are saying in this statement. The fatwa will work to provide a better environment for safety and security.”

However, Raza noted that British Muslims were still permitted for to travel to Syria on humanitarian missions.

“We oppose Isis [Islamic State], and anybody who is supporting Isis will see our position,” said Mufti Abdul Kadir Barakatullah, a member of the UK Shariah Council. “Fighting is only exacerbating and prolonging the suffering.”