Prime Minister Rishi Sunak has written to the family of a jailed British-Egyptian writer as he demanded an end to the detainee’s “unacceptable treatment” in prison in Cairo.

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 time for your brother’s case; 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.”

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Mr Sunak told El-Fattah’s family his predecessor Boris Johnson 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.”