Dr Abbas Khan travelled to the city of Aleppo last year to help civilians
British doctor Abbas Khan has been "in effect murdered" by the Syrian authorities just days before his expected release from jail, Foreign Office Minister Hugh Robertson said.
He said the death of the 32-year-old orthopaedic surgeon from south London was "at best extremely suspicious".
Dr Khan was arrested last year when he arrived in Aleppo to help civilians.
Syria's Deputy Foreign Minister Faisal Mekdad said Dr Khan committed suicide using his pyjamas to hang himself.
Dr Khan's family had been told he would be released on Friday following an order from the Syrian President Bashar al-Assad.
His brother, Afroze Khan, said when his mother went to see her son in Damascus on Monday she was told he was dead.
Mr Robertson said the government was seeking "urgent clarification" about what happened to Dr Khan, originally from Streatham.
He said "We can't at the moment be absolutely certain about the circumstances in which Dr Khan met his death, but what is clear is that he went to Syria on a humanitarian mission, was imprisoned by the Syrian authorities and met his death while he was in prison in circumstances that are at best extremely suspicious.
"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 who were injured during their civil war."
The Respect MP George Galloway, who has liaised with the Syrian authorities and was due to collect Dr Khan on Friday, said the death was "murder most foul" and it was "inconceivable that he committed suicide".
The MP for Bradford West said he believed somebody within the regime killed Dr Khan to defy President Assad's decision to release him.
Dr Khan was expected to be released on Friday
BBC correspondent Paul Wood, reporting from Beirut, said Dr Abbas could have been a casualty of a power struggle between the Syrian president and his own security services.
He said: "Only President Assad could have ordered the doctor's release - if he did so it may well have been a goodwill gesture in the run up to peace talks that Britain is helping to organise.
"Some in the regime perhaps do not want those talks to succeed."
Afroze Khan said: "Life's been turned upside down. We are a very close knit family. We have been through this ordeal for 13 months now and are in utter despair really.
"But we are also proud that he died doing something that he believed in - helping people who were in desperate need."