Syria's deputy foreign minister Faisal Mekdad had said 32-year-old orthopaedic surgeon Dr Abbas Khan committed suicide, but British junior Foreign Office minister Hugh Robertson said the regime of president Bashar al-Assad was responsible for the death of the UK national.
Robertson said, "There is no excuse whatsoever for the treatment that he has suffered by the Syrian authorities who have in effect murdered a British national who was in their country to help people injured during their civil war."
Khan's family said they were "shocked and devastated", as Syrian authorities had promised to release him this week but then days later told the family he was dead.
Abbas' brother Shahnawaz said, "The fact that this individual was out there helping the humanitarian effort and has been held for 13 months against his will without a charge or a trial or access to a lawyer, and they have offered very little assistance, placated us throughout."
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"We had politicians due to go over to Damascus to go and get him. He knew about that. He was in high spirits. We thought he was coming home this Friday," he said.
Respect Party MP George Galloway, who has liaised with the Syrian authorities and was due to collect Khan on Friday, said the death was "murder most foul" and it was "inconceivable that he committed suicide".