Britain accused Syria on Tuesday of effectively murdering a surgeon who was detained after volunteering to work at a hospital in the war-torn country and then died in jail.
Abbas Khan, a 32-year-old orthopaedic surgeon from south London, travelled to the northern city of Aleppo last year to help civilians but was captured by the Syrian regime.
His family said on Tuesday that the father-of-two had died in detention, just days before he was due to be freed and handed over to a British lawmaker.
Syrian officials reportedly said Khan had committed suicide, but British junior Foreign Office minister Hugh Robertson said the Assad regime was responsible.
"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," Robertson told the BBC.
"The fact remains that he went to Syria to help the people of Syria who were affected by the 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 had died.
"We thought this Friday was to be the day. The deputy foreign minister of Syria called my mum to say they were going to let him go," his sister Sara Khan told Britain's Sky News.
"We were so happy, we started decorating and preparing for his return."
But the family then heard that he had been taken away from the prison as part of the final stage of his release, but they were sceptical.
"He went on Friday and on Monday we got a call from a security service official to say he was dead. Syrians are calling it a case of suicide -- the statement released a ridiculous story of how he killed himself," Sara Khan said.
Khan's family had been unaware of his plight for eight months until his mother Fatima travelled to Damascus over the summer and managed to track him down.
Khan reportedly wrote to British Foreign Secretary William Hague earlier this year describing how he had lost around half his body weight in the squalid conditions of his detention, during which he had been forced to beat other prisoners.
A 'deeply troubling death'
The family criticised the Foreign Office's handling of the case, saying they had taken too long to help.
"We are devastated, distraught and we are angry at the Foreign Office for dragging their feet for 13 months," Khan's brother Afroze told the BBC.
A Foreign Office spokesman said it had sought consular access to Khan on several occasions via Russian and Czech diplomats in Damascus, but all its requests had been ignored.
Firebrand lawmaker George Galloway -- a vocal critic of the western policy on supporting Syrian rebels, having bitterly opposed the Iraq war in 2003 -- said he had been due to fly out to Syria within days to bring the doctor home.
"Last week I received a call from the (Syrian) foreign minister telling me that the president had asked him to contact me to come to Damascus to bring Dr Khan home before Christmas," Galloway said in a statement.
"Obviously this had to be kept confidential but the family were kept fully informed. I was in the process of booking a flight for this Friday when I got the appalling news."
The British-based Syrian Observatory for Human Rights rejected the suicide claim, saying Syrian authorities were notorious for torturing prisoners.
"The Observatory believes there is an overwhelming likelihood he died of torture, because there are hundreds of similar cases in which the regime says a prisoner committed suicide when in fact they were tortured to death," founder Rami Abdel Rahman told AFP.
Rights group Amnesty International said this was "yet another deeply troubling death in custody in Syria" and called on the British government to "denounce" his death.