17-year-old Hassan Munshi, believed to be of Pakistani- origin, and his friend have vanished from the West Yorkshire town of Dewsbury last week, becoming the latest young British Muslims to have left their homes and families for the jihadist insurgency, 'The Times' newspaper reported.
Hassan's brother, Hammaad Munshi, was 15 when he was arrested by counter-terrorism police in 2006 and later convicted over his role in a plot to murder non-Muslims.
It is understood that Hassan and his friend Talha Asmal, a student who was a neighbour of the Munshis and is also thought to be 17, have been missing for several days.
Their families grew worried and contacted police when they were unable to make contact with their mobile phones.
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Talha is believed to have told his family that he would be away for a few days on a school trip.
Efforts to trace the missing Dewsbury teenagers initially focused on the Turkish side of the Syrian border, but British authorities are understood to believe that both are now in Syria.
Scotland Yard are yet to confirm the reports but 'The Times' report claims that the missing teenager is the younger relative of Hammad Munshi, who was jailed over his terror leanings.
A judge at Old Bailey court in London said while sentencing the teenager in 2008 that his head had been filled with "pernicious and warped ideas" which led to his involvement in a plan to kill non-believers.