Al-Britani, who is believed to have been called William Hasmou Clinic was reportedly killed in Hawigat Saqr, near the eastern Syrian city of Deir ez-Sor, in a Syrian government air strike. He is believed to be the eighth Briton killed in the conflict.
He was very active on social media and wanted fellow Britons to join him in jihad. He said: ‘I’m sure we have more lions roaming around the UK. Get yourself over here.’
In July, he appeared in a video, claiming to have witnessed a ‘miracle’ during a battle in which he saw angels. He claimed that during a 22 hour battle, ‘Allah sent his angels upon angels upon angels to aid the believers in battle.’
In one of his Tweets, al-Britani said: ‘From the streets of London to the dirt roads of Jihad.’
It appears he might have got married in Syria and had a daughter, whom he referred to as his ‘little princess mujahidah’.
The Foreign and Commonwealth Office is currently investigating reports of al-Britani’s death.
A spokeswoman said: ‘We are aware of reports of the death of a British national in Syria.
‘The UK has advised for some time against all travel to Syria, where all UK consular services are suspended. As we do not have any representation in Syria, it is extremely difficult to get any confirmation of deaths or injuries and our options for supporting British nationals there are extremely limited.’
It is understood that Bashar al-Assad’s airforce bombed Deir ez Zur earlier today. It has been reported that al-Britani was killed alongside a fellow fighter called Ahmad Bakir.
Charlie Winter, a researcher with anti-extremist think thank the Quilliam Foundation, tweeted: ‘Multiple (as yet unconfirmed reports that another Briton – Abu Abdullah al-Britani – died fighting for IS in #Syria.’
Shiraz Maher, from the International Centre for the Study of Radicalisation in London, added: ‘Unconfirmed reports that a British foreign fighter, William Hasmo Clinic, aka Abu Abdullah, has died while fighting for IS in Syria.’
Another Twitter account, @Raqqa-SI, which is anti-Isis, posted: ‘Today #ISIS mourns William Hasmo Clinic a British known as Abu Abdullah in Hwejet Saqer area Dier AlZour #Syria’.
Other British fighters who have been killed in the fighting include Abdullah, 18, and Jaffar, 17, Deghayes from Brighton, East Sussex.
Jaffar, 17, is believed to have died last month trying to overthrow dictator Assad’s government.
His brother, Abdullah, 18, died in Latakia province in April after leaving the UK in January to reportedly take up arms with al-Nusra.
Their older brother Amer Deghayes, 20, is fighting for al-Nusra, an al-Qaeda affliated group, against the Bashar al-Assad regime.
He said their deaths had made him more determined to carry on fighting until he is killed, revealing that he has no desire to return to the UK.
‘I have promised Allah that I will stay on the way of jihad until I get killed,’ he told ITV News.
He describes martyrdom as the ‘greatest success a person can attain’, adding: ‘It [his brothers’ deaths] makes me more determined to get what they got.’
The Deghayes brothers are the nephew of Omar Deghayes, who was held by the US as an enemy combatant at Guantanamo Bay detention camp from 2002 to 2007 after he was arrested in Pakistan.
It emerged this month that a fourth man from Portsmouth, Hampshire – Muhammad Mehdi Hassan, 19 – died fighting in Kobane, the scene of fierce fighting between Kurds and the militant group.
Three others from the same city – Iftekar Jaman, 23, Mamunur Roshid, 24, and Muhammad Hamidur Rahman, 25 – have also been killed after travelling there in October last year.
In January alone, 16 people were arrested on suspicion of terrorism offences related to Syria compared with 24 arrests in the whole of last year.
Others who have died include one man suspected of carrying out a suicide attack. Abdul Waheed Majeed is believed to have driven a lorry to a jail in Aleppo before detonating a bomb in February.
The 41-year-old married father-of-three, who was born and raised in Crawley, West Sussex, left Britain in 2013, telling his family he was going on a humanitarian mission to Syria.
By Tom Wyke