David Cameron has written to the mother of a British doctor who died in custody in Syria, describing his death as "a sickening and appalling tragedy".
Dr Abbas Khan, 32, was on the verge of being released when his family were told of his death this week.
His relatives have said he was the victim of a political murder, but the Syrian government have called his death suicide.
Dr Abbas Khan
His body was flown back to the UK today and will undergo a post-mortem examination.
In a letter dated December 20, the Prime Minister told his mother, Fatima Khan, that he and his wife Samantha were "so very sorry" to hear of her son's death.
"I know from my own experience of losing a child that words are of little comfort at this terrible time but please know that you are in our thoughts," he wrote.
"Abbas' death is a sickening and appalling tragedy and it is right that the Syrian regime should answer for it.
He branded the regime's treatment of Dr Khan "despicable" and claimed it was "utterly unacceptable" that the UK were not able to support him.
In a hand-written ending, he added: "You are in our thoughts and prayers."
The orthopaedic surgeon from London was captured in November 2012 in the ancient city of Aleppo after travelling from Turkey to help victims of hospital bombings.
Dr Khan's brother Afroze said he was "grateful" that Mr Cameron had condemned the Syrian regime but insisted that the Prime Minister must now "follow through" and use every possible method to bring to justice "those who have murdered my brother".
He told the BBC: "We are very angry at the failures of the British government at securing the release of my brother and now that he is dead we want to see the British government act decisively to respond to the death of a British citizen at the hands of a foreign regime."
Mr Khan added: "There must be a firm response to such a callous and cruel act."
Nabeel Sheikh, solicitor for Dr Khan's family, issued a statement which read: "Dr Khan's family would like to thank everyone for their help and support. The family are relieved that his body has been repatriated. Dr Khan is being transported by ambulance to the Coroner's Court in Walthamstow for the purposes of a post-mortem.
"The family hopes that all relevant tests as deemed necessary are carried out so as to ensure the post-mortem is concluded without delay and the body laid to rest as soon as possible.
"The Home Office is organising the process but, as a matter of course, Scotland Yard is actively involved given the circumstances surrounding Dr Khan's death are highly suspicious.
"Our thoughts are with the family. The immediate priority is to give the family privacy and enough support as necessary to help them through this difficult time."
The International Committee of the Red Cross (ICRC) said it transported the body from Damascus to the Lebanese capital Beirut, where it was received by Mrs Khan and British officials.
Mrs Khan, who has ''110%'' refuted claims that he committed suicide, broke down in tears when the coffin arrived.
''The national security intelligence of Syria, they killed him!'' she screamed. ''They're murderers!''
Mrs Khan has categorically denied claims made by Syria's deputy foreign minister Faisal Mekdad that he had killed himself.
In the last few days the family revealed a letter in which the doctor expressed his optimism at being released, and his hopes of being home in time for Christmas.
The full text of David Cameron's letter:
Dear Mrs Khan,
Samantha and I were so very sorry to hear about the tragic death of your son, Abbas, in Syria. I know from my own experience of losing a child that words are of little comfort at this terrible time but please know that you are in our thoughts. I hope that, in time, you will remember all the good things about Abbas' life.
You have shown incredible courage and fortitude throughout the last 13 months. I cannot begin to imagine how devastating losing Abbas must be, especially at a time when it seemed as if the regime might finally release him. I hope that you can draw some comfort from the out pouring of support you have received, and that you have the space and time to grieve for him, as well as celebrate, with pride, the fact that he dedicated so much of his life helping others, including some of the most vulnerable people in the world.
The Foreign Secretary has been briefed regularly on Abbas' case and I have asked the Foreign Office to do all they can to bring Abbas' body home as soon as possible.
Abbas' death is a sickening and appalling tragedy and it is right that the Syrian regime should answer for it. Their despicable treatment of him and refusal to engage with us or the Czechs to enable us to support him is utterly unacceptable. We will continue to press for those responsible to be held to account.
Again, please accept my deepest condolences. Our thoughts are with you.
(Handwritten) You are in our thoughts and prayers.
Yours sincerely,
David Cameron