An Al-Jazeera journalist held in Egypt since August is in a "critical" condition and could slip into a coma after more than 100 days on hunger strike, his brother said on Tuesday.

Abdullah El-Shamy was arrested on 14 August when police dispersed supporters of ousted Islamist president Mohamed Morsi in Cairo, which led to clashes in which hundreds were killed.

El-Shamy, who works for the Qatar-based network, went on hunger strike in January to protest his detention and has yet to face trial.

He is suffering from "anaemia, the start of kidney failure, low blood pressure and hypoglycaemia," his brother, Mosaab, told AFP.

"He is at a critical stage and needs to be transferred to hospital," Mosaab said, providing AFP with a copy of a blood test on his brother from last week.

On Monday, the doctor who reviewed El-Shamy's test results told Al-Jazeera English that the journalist's condition was life-threatening and he could "die within a few days."

At the start of his hunger strike, El-Shamy drank juice and other sugary drinks but has been taking only water for the past month, his brother said.

And he has shed around 40 kilogrammes (88 pounds) over the past 112 days, he added.

"He can go into a coma if he doesn't take perfusions (drips) and if his blood sugar remains low... It is the start of the most dangerous period," he added.

Mosaab added that his brother had recently been moved from his prison cell to an unknown location. "Surely he is still in jail but we don't know where and we cannot communicate with him."

El-Shamy's lawyer, Shaaban Saeed, told AFP he was accused of joining a "terrorist group" and spreading false news.

"My client is paying the price for working for a channel that opposes the ruling regime," Saeed said.

The interim authorities have been incensed by Al-Jazeera's coverage of their crackdown on Morsi's supporters since July when the army ousted him amid mass protests.

On 3 May, a court remanded El-Shamy in custody for another 45 days, and he appeared in court looking gaunt.

"I haven't seen my lawyer. We are 15 people in a cell of 12 square metres (130 square feet)," he told reporters from the dock.

Last week, El-Shamy wrote in Arabic from his prison cell that a guard "was trying to convince me to put an end to my hunger strike, saying that it was 'a sin' one time, or that 'Egypt needed me' another time. So I replied: 'Egypt needs me to be in prison?'"

"He was trying to be friendly by saying he would refer my case to the prosecutor and to the court as if that had not been done already," he added in the letter published by Al-Jazeera.

His wife Jihad Khaled, meanwhile, is also on hunger strike, her mother Houda Abdelmoneim told AFP.

"Jihad began her hunger strike from 14 March to express solidarity with her husband," Abdelmoneim said, adding that her daughter fell unconscious on Monday and was taken to a hospital as she too had been taking "only water."

Three other Al-Jazeera journalists, who work for the network's English-language channel, are held in Egypt and are on trial for defamation and supporting Morsi's banned Muslim Brotherhood.

Their trial of Australian Peter Greste, Egyptian-Canadian Mohamed Fadel Fahmy and Baher Mohamed, and 17 other co-defendants, has sparked an international outcry. The case has been adjourned until 15 May.

Al-Jazeera strongly denies the accusations made against its staff and has called on the Egyptian authorities to release them immediately.