"There are Britons who upon wanting to leave have been threatened with death, either directly or indirectly," the Guardian reported, citing a source with extensive contacts among Syrian militant groups.
The report comes after the killing of another young Muslim from Portsmouth on the frontline in Syria, the fourth to die from a group of six men known as the "Pompey lads' who travelled together to fight for ISIS.
19-year-old Muhammad Mehdi Hassan is understood to have died on Friday during the ISIS offensive to capture the Syrian border city of Kobani.
Hassan was part of a group of five calling themselves the Britani Brigade Bangladeshi Bad Boys. ISIS is an al-Qaeda splinter group and it has seized hundreds of square miles in Iraq and Syria. Abu Bakr al-Baghdadi, leader of the militant group, declared himself as Caliph renamed the ISIS as Islamic State.
Al-Qaeda has distanced itself from the group, chiding it for its lack of teamwork in its aggressive, brutal expansion. The former Guantanamo Bay detainee Moazzam Begg confirmed that he was also aware of dozens of British men keen to return to the UK but they were trapped in Syria and Iraq, in effect held by a group they wanted to leave, the paper said. Begg said he knew of more than 30 who wanted to come back.
They had travelled to join rebels fighting the Syrian regime of President Bashar al-Assad but had subsequently become embroiled with ISIS, some for language reasons ISIS had more English-speaking members.
"When it becomes solidified as an Islamic State, a caliph and you swear allegiance, thereafter if you do something disobedient you are now disobeying the caliph and could be subject to disciplinary measures which could include threats of death or death," said Begg.
There are an estimated 500 Britons who have travelled to fight in Iraq and Syria. Overall, 24 Britons are believed to have died after travelling to fight in the bloody civil war.