A British doctor has died in a Syrian jail, his family said on Tuesday.
Abbas Khan, an orthopaedic surgeon from south London, was arrested within 48 hours of arriving in Syria to offer his services in rebel-held Aleppo.
His family was told he would be released this week, his brother told the BBC, but when his mother went to visit him in Damascus, she was told he had died.
"Syrian authorities have in effect murdered a British national who was in their country to help people," said Hugh Robertson, a minister at the UK Foreign Office.
A Syrian government official said Khan, a 32-year-old father of two, had committed suicide, reported the BBC.
But his brother said that was impossible, given that he was preparing to go home with his mother.
"He was happy and looking forward to being released," Afroze Khan said. "We are devastated, distraught and we are angry at the Foreign Office for dragging their feet for 13 months."
MP George Galloway, who was due to collect Khan on Friday from the Syrian authorities, agreed it was "inconceivable that he committed suicide".
The foreign office said it had frequently sought consular access to Khan, as well as information on his detention, both directly and through the Russians, Czechs and others.
Britain closed its embassy in Damascus in February 2012.
"These requests have consistently been ignored," it said in a statement early on Tuesday.
In letters to British Foreign Secretary William Hague, Abbas Khan said he had been tortured in detention and kept in isolated, squalid conditions.
A second Briton, 23-year-old Ifthekar Jaman, was reported to have been killed in Syria at the weekend after joining an armed rebel group opposed to President Bashar al-Assad.
The foreign office said it was aware of the report and was seeking clarification, but added again that its options for supporting Britons in Syria were "extremely limited".
"We continue to advise against all travel to Syria," it said.
After security forces repressed peaceful protests against more than 40 years of Assad family rule in 2011, an armed revolt ensued with an increasingly sectarian element. Well over 100,000 people have been killed and millions forced from their homes.