Jailed British-Egyptian activist Alaa Abdel Fattah has stopped drinking water as he steps up his hunger strike to coincide with the start of the COP27 summit, his sister has said.
Calls for his release escalated after the climate summit opened in Sharm el-Sheikh in Egypt on Sunday.
The 40 year old has consumed just 100 calories for more than 200 days to push Egypt to allow him UK consular access.
UK PM Rishi Sunak has said he will raise the issue at the COP summit.
Abdel Fattah, a key activist in the 2011 Arab Spring, is currently serving a five-year sentence for spreading false news.
His sister, Sanaa Seif, has warned that her brother’s hunger and water strike may mean he could die before the end of the summit.
Speaking to Sky News, she urged the British government to be “responsible for getting us proof of life”.
Mr Sunak wrote to Abdel Fattah’s family and said he would raise his imprisonment with the Egyptian government and reply again by the end of the summit.
He said the activist’s case is “a priority for the British government both as a human rights defender and as a British national”.
Ms Seif, a 28-year-old human rights activist who has served three prison sentences in Egypt herself on charges that fellow activists condemned as bogus, has been protesting outside the Foreign Office in London along with family members for her brother’s release.
She expressed concerns that Downing Street’s engagement with the Egyptian president Abdel Fattah El-Sisi would come too late.
Amnesty International Secretary General Agnes Callamard said Abdel Fattah “must be released” and warned that he may only have 72 hours to live.
“Let’s be very clear, we’re running out of time,” she said in Cairo on Sunday. “So if the authorities do not want to end up with a death they should have and could have prevented, they must act now.
“If they don’t, that death will be in every single discussion in this COP.”
Abdel Fattah’s aunt, Ahdaf Soueif, told the BBC that the summit could be his last chance to be saved and to be released.
She urged Mr Sunak to secure her nephew’s release.
“It means we really only have a few days,” she said. “None of us have any reason to believe that the regime will ever let him go.”
“He has known for a while that he’s had enough, that he cannot live like this. And this is now his opportunity and all of us really to bring matters to a head.
“He’s betting on us and on the community inside Egypt that wants him released and on the international community that’s making a noise for him.”
She said the UK government could use its influence to have him released.
“This is all in the hands of the British government to facilitate… it would be very difficult for the UK to do business as usual with Egypt unless this case is resolved.
“And I think if the British government is serious and if Rishi Sunak says this convincingly, Alaa will be on a plane to London.”
Abdel Fattah played a key role in the protests that toppled the former Egyptian leader, Hosni Mubarak, from power in 2011.
He has been in jail for nine years and was sentenced to a further five years in 2021 on charges of “broadcasting false news” – a charge human rights groups condemned as spurious.
He received British citizenship in December 2021 through his London-born mother.
Human rights groups have said he is one of an estimated 60,000 Egyptian political prisoners and have accused the government in Cairo of trying to “greenwash” its repressive reputation through its hosting of the climate summit.
The Egyptian government has insisted there are no political prisoners in the country.