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.
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.
The International Committee of the Red Cross (ICRC) said it transported his body from Damascus to the Lebanese capital Beirut, where it was received by his mother Fatima 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!"
The ICRC said it expected the British Embassy to fly Dr Khan's body to London "rapidly".
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.
A convoy of British doctors headed for Syria despite the death of Dr Khan.
The Observer reported aid groups vowed not to be deterred as a fleet of more than 40 ambulances carrying medical volunteers and supplies left on an eight-day journey to Syria.
According to the newspaper, several of the vehicles had "RIP Dr Abbas Khan" written on the side.
Dr Shameela Islam-Zulfiqar, from Manchester, said: "It's really tragic that we lost Dr Khan, but even his family have said there are hundreds dying in Syria every day and so many doctors and medical staff just want to help.
"People have been asking, 'why are we going?' The question is why aren't we doing more? The work of UK charities is a drop in the ocean, but I'd rather be part of that than do nothing.
"The timing of Dr Khan's death is very deliberate by the regime. They know the holiday season means the aid convoys will be coming and it was a very symbolic act - don't come or look what we will do."
Dr Islam-Zulfiqar will be on board the convoy, which is funded by the Worcester-based charity Al Fatiha Global and the Aid4Syria campaign, the Observer reported.