Ex-student Sufiyan Mustafa, 22, travelled over 2,000 miles to team up with rebel groups, intelligence sources claim.
He is said to have stayed a shadowy “ghost” since arriving by merging into the background of the bloody conflict, The Sunday People.
But it is thought he has played an active role in fighting in the north of the country and was involved in the siege of Aleppo.
A source confirmed: “He has been part of military operations.”
Sufiyan is the sixth of eight children fathered by notorious Egyptian-born Hamza, 58, now serving life in jail in America for terror offences.
Before heading to Syria, Sufiyan studied at London’s Westminster University, which was also attended by IS executions beast Mohammed “Jihadi John” Emwazi.
The source added: “Sufiyan has no sign of a criminal record with the British Government. He was at university before leaving for northern Syria.
“He has managed to stay a ghost since he set foot in Syria but it’s thought he has been with rebels in the north.”
Experts believe 800 Brits are among 27,000 foreign fighters who have flocked to Iraq and Syria to join IS since fighting broke out in 2011.
Sufiyan’s father is infamous for having hooks in place of his hands and only one eye, injuries he has variously claimed were caused by a landmine in battle or an accident while working with explosives.
He spent years living in London before he was extradited to the US and convicted of offences related to 1998 hostage-taking in Yemen which left three Britons and an Australian dead. In 2015 he was branded “evil” by a judge in New York as she jailed him for 11 crimes including plotting to set up a US terror training camp.
Last year it emerged he had launched a legal fight to be transferred from his high-security US jail back to Britain.
Hamza’s family caused uproar after it emerged they were allowed to go on living in a £1.25million, five-bed house at taxpayers’ expense despite his conviction.
It is said his wife and kids claim more than £650 a week in state handouts and have had the house refurbished twice, with one makeover alone costing £25,000.
Hammersmith and Fulham Council, the authority which covers the area where they live, is unable to evict them because there is no time limit on the tenancy.
Two of Hamza’s sons and a stepson have also been in jail for offences ranging from a bomb plot to a robbery.
When the Sunday People called at the Hamza family home a woman refused to open the door to us or comment.