A fourth man from Portsmouth who went to fight in Syria for Islamic State (IS) is believed to have been killed.
Mehdi Hassan, 19, travelled to the country with a group of four other men in October 2013.
A photo of his body emerged on Twitter on Friday and his family confirmed earlier to their local mosque they had received news of his death.
Iftekar Jaman, Mamunur Roshid and Hamidur Rahman have previously been killed in the fighting.
In a statement, the Foreign Office said it had not received any reports about Mr Hassan's death but was "aware of reports about the death of a British national in Syria".
'Well mannered boy'
Chairman of the Portsmouth Jami Mosque, Abdul Jalil, said: "It has been confirmed with the family that he has died. Right now they are very upset.
"I am saddened and again shocked for the community about this news."
It is thought he died in Kobane.
A family friend, who did not want to be named, told the BBC: "Mehdi Hassan was a polite well mannered boy and always spoke to elders with respect.
"He had a good family upbringing. It's a shame, he would have been a valuable member to Portsmouth society."
On 17 October, a tweet from an account believed to be linked to Mr Hassan said: "Between 20-40 us strikes daily in ayn al arab [the Syrian town of of Kobane]. Alhamdulillah they are spending $10's of billions...against themselves.
Emma Vardy, Home Affairs correspondent, BBC South:
"Earlier this year Mehdi's mother had herself travelled to the Turkish border to try to find out more about what led her son to leave his home behind.
His family said he used to phone home when he could, sometimes every couple of months or so.
Mr Hassan's death will not only be felt within his community, but also on social media.
He accumulated a considerable online following during his time in Syria and often posted tweets about his time in the conflict.
At one time, he hosted a question and answers session on Facebook for friends back home.
He discussed everything from what the Syrian food was like, to the prospect of facing his own death.
Amongst extremist fighters online, news of a death is not mourned, but celebrated. They are said to have achieved their aim of 'martyrdom'.
But here in Portsmouth it will be felt very differently.
The Bangladeshi community now has four families who have lost their sons in Syria.
Many here simply feel great sadness, that this group of young, once well-liked men, conspired together to chose this path."
On Tuesday, the mosque confirmed Mr Roshid, 24, had died. He left the UK with Mr Hassan, Mr Rahman, Assad Uzzaman, and Mashudur Choudhury.
Choudhury returned to the UK after a few weeks and was arrested at Gatwick Airport.
Mr Uzzaman is still believed to be in Syria.
Britain's most senior police officer, Metropolitan Police Commissioner Sir Bernard Hogan-Howe, has said at least five Britons are travelling to Iraq and Syria to join IS every week.
An estimated 500 are already fighting with the group.