Jail staff are threatening to force-feed Putin critic Alexei Navalny, who has lost 15kg (33lbs) in weight since he was taken to the prison last month, according to his allies.
The Russian opposition leader, 44, began a hunger strike at penal colony number 2 in Pokrov at the end of March to highlight what he said was the authorities' refusal to give him proper treatment for his back and leg pain.
He wants to be helped by a doctor of his choice to check his condition, but staff at IK-2 have declined his request.
Authorities claim they have offered him the correct treatment but say he has refused it.
Watch: Security stepped up at Navalny prison
Navalny, whose weight has reportedly fallen to 77kg (170lbs), was moved to a prison clinic earlier this month after complaining of a high temperature and a bad cough.
But he has now been discharged from the medical facility, according to his Twitter account, which is updated by his allies.
It said: "Given the severity of the hunger strike, the (prison) administration is threatening on a daily basis to start force-feeding."
Navalny had already lost 8kg in the jail, 60 miles from Moscow, before starting his hunger strike, his allies said on 1 April, blaming the weight drop on guards deliberately depriving him of sleep.
Prison authorities deny depriving him of sleep and previously said his condition was satisfactory.
Jail officials have not yet commented on the latest claims.
Navalny, one of the most outspoken critics of President Vladimir Putin, was detained in January as he returned to Russia from Germany, where he spent five months recovering from a nerve agent poisoning.
He has blamed the Kremlin for targeting him with novichok - an allegation that Russian authorities reject.
Navalny was jailed in February for two years and eight months for parole violations he says were trumped up.
Watch: Jailed Kremlin critic on hunger strike over lack of proper medical treatment