"Emma Smart, 44, from Weymouth, Dorset, who has today been in prison on hunger strike for 13 days, was moved out of her cell onto the hospital wing at HMP Bronzefield on Friday. The prison is becoming increasingly concerned about her health", Insulate Britain said in a statement.
On 17 November, a London High Court judge found Smart and another eight climate activists guilty of contempt of court for breaking the National Highways' M25 injunction and sentenced them to between three and six months in jail.
The judge also ordered the nine to pay costs of 5,000 pounds ($6,720) each and said all the defendants should serve at least half of their sentence.
A further group of nine people from Insulate Britain have been summoned to appear at the High Court on 14 December and if found guilty they could be subject to unlimited fines, seizure of assets and prison sentences of up to two years.
The campaign group has been staging sit-down protests on the M25 highway encircling Greater London and other major roads across England since September despite the court issuing four injunctions banning them from the roads.
According to Insulate Britain, which is demanding that the UK government insulate and retrofit homes to tackle climate change, 8,500 deaths in an average British winter are the result of cold homes.