Jaffar Deghayes, 17, is believed to have died at the weekend after leaving home near Brighton earlier this year in a bid to overthrow dictator Bashar 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 Jabhat al-Nusra, an al-Qaida-affiliated group.
Yesterday the pair’s brother, Amer, 20, who also travelled to Syria, relayed news of the death of Jaffar to their father, Abubaker Deghayes, back home in Saltdean, East Sussex.
Mr Deghayes said: “Amer sent me a message via the internet. All I know is that (Jaffar) was fighting against Assad and was killed in battle.
“I don’t know much else. I can only hope and pray to God to accept him and have mercy on him.”
The Deghayes brothers are the nephew of Omar Deghayes, who was held by the United States as an enemy combatant at Guantanamo Bay detention camp between 2002 and 2007 after he was arrested in Pakistan.
Following the death of Abdullah, counter-terrorism officers raided the Deghayes’ family home in May and seized material under the Terrorism Act 2000.
Abubaker Deghayes has previously said that his three sons could have helped the cause in Syria from the UK by lobbying the Government.
But this year Amer Deghayes said he had no intention of returning to Britain, adding: “My work here is not done. I came here to give victory to the people and make sure that they receive justice, and we still haven’t reached the goal yet.”
He added: “I’m in the Syrian civil war because I believe it’s my duty to fight here... The Muslim nation is like one body.
“If one part complains, the other parts react, so I don’t see it as a Syrian conflict. I see it as an Islamic conflict.”
Counter-terrorism investigators have expressed concern about aspiring British jihadis travelling to Syria and becoming radicalised.
It emerged this month that a fourth man from Portsmouth, Hampshire – 19-year-old Muhammad Mehdi Hassan – died fighting in Kobani.
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, 41, is believed to have driven a lorry to a jail in Aleppo before detonating a bomb in February. The 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.
The Staffordshire father of a woman who is believed to have gone to Syria with her 14-month-old son has spoken of his shock.
Tareena Shakil’s father Mohammed said he is concerned for his grandson’s welfare after it emerged his daughter had travelled to Turkey and crossed into Syria, despite telling her family she was going on holiday to Spain.