Rishi Sunak has written to the family of a British-Egyptian writer imprisoned in Cairo, as the activist's sister urged the prime minister to get "proof of life" from Egyptian authorities.
Alaa Abd El-Fattah has been kept behind bars in Egypt for most of the past decade and was sentenced in December after being accused of spreading fake news.
His sisters, Sanaa and Mona Seif, along with other family members, are protesting the imprisonment of the pro-democracy writer and activist.
They started a sit-in in Whitehall on 18 October and intend to continue it until the COP27 conference.
In an official letter, shared by the prisoner's family with Sky News, Mr Sunak said he had been "following Alaa's case closely and was concerned to hear about his deteriorating health".
He added: "I appreciate this must be an extremely painful time for your brother and the whole family; he remains a priority for the British government, both as a human rights defender and as a British national.
"Ministers and officials continue to press for urgent consular access to Alaa as well as calling for his release at the highest levels of the Egyptian government."
Mr Sunak told El-Fattah's family his predecessor Liz Truss had raised the case with Egyptian President Abdel Fattah al Sisi.
"I will continue to stress to President Sisi the importance that we attach to the swift resolution of Alaa's case, and end to his unacceptable treatment," he wrote.
"The UK's attendance at COP27 is another opportunity to raise your brother's case with the Egyptian leadership.
"I would like to thank you again for writing and reassure you that the government is deeply committed to doing everything we can to resolve Alaa's case as soon as possible."
'He could die during COP27'
The letter from Mr Sunak comes as the activist's family makes increasingly desperate pleas for help.
Mr El-Fattah has been on a "spiritual hunger strike" for around 200 days, and recently told his family he would "escalate" this to no longer drinking water.
Speaking to Sky's Sophy Ridge, his sister Sanaa Seif said she feared her brother would die while Mr Sunak is in Egypt for COP27.
She said when she last saw him in August, he "already looked very, very frail" and "his eyes were sunken".
Ms Seif warned that by the next time she is able to see Mr El-Fattah on 16 November, it could be "too late".
Her family also wants to know that Mr El-Fattah is still alive, urging the prime minister to get "proof of life" from the Egyptian authorities.
She alleged that her brother was being treated like a "terrorist" and denied basic rights, and that he had not seen his young son in more than a year.
In a plea to the government, she said the issue was "very urgent" and that she hoped politicians were not just paying "lip service" to her family.