Palestinians attend a protest in solidarity with Palestinian prisoners in Israeli jails in Hebron, in the occupied West Bank, on May 25, 2023. AFP
Walid Daqqa, a Palestinian serving time for the kidnap and murder of an Israeli soldier, has cancer, the advocacy group said.
"He is now on artificial ventilation and his lungs and kidneys are in great distress," it said in a statement.
A spokeswoman for the Israel Prison Service told AFP that Daqqa is currently at Shamir Medical Centre in central Israel.
Daqqa identifies as Palestinian although he holds citizenship from Israel, which would consider him Arab-Israeli.
Palestinians gathered in the West Bank city of Hebron on Thursday for a rally in solidarity with Daqqa and other Palestinians held by Israel.
The Palestinian Prisoners' Club said it delivered a letter to the International Committee of the Red Cross (ICRC) in Hebron and nearby Bethlehem, calling for the organisation to intervene in Daqqa's case.
An ICRC spokeswoman told AFP it had received the letter and the organisation had previously visited Daqqa.
Daqqa, 61, was diagnosed in December with a rare form of blood cancer -- myelofibrosis -- the Palestinian Prisoners' Club said.
The organisation said he was also diagnosed with leukaemia in 2015 and is currently due to be freed in March 2025.
Judicial authorities in Israel did not immediately respond to an AFP request regarding Daqqa's appeals for early release on medical grounds.