Fatlum Shalaku, 20, of Ladbroke Grove, was involved in strikes on government buildings in Ramadi, while his older brother Flamur, 23, is said to have died on an Iraqi battlefield in March, The Sunday Times reported.
Before joining Isis, the pair attended Holland Park School, once renowned for being the place where Labour politicians Tony Benn and Anthony Crosland sent their children.
Its more recent alumni include Mohammed Nasser, 21, and Hamzah Parvez, 22, who appeared in a propaganda video last year urging British Muslims to “come to the land of jihad”.
Two other former Holland Park students, Mohammed al-Araj, 23, and Nassim Terreri, 25, are also thought to have been killed in Syria. Colin Hall, the headmaster, has said in the past the school has “a very strong line on zero tolerance to any kind of fundamentalism”.