The climate change protest group said in a statement released on Monday morning that the 44-year-old, from Weymouth in Dorset, was moved from her cell at the Surrey prison after staff became “increasingly concerned about her health”.
Ms Smart was sentenced to four months in prison for contempt of court on 17 November for breaking an M25 injunction after protesters blocked a number of roads across the country.
Ms Smart was jailed alongside Ben Buse, 36, Roman Paluch-Machnik, 28, Oliver Rock, 41, Tim Speers, 36, and James Thomas, 47, for taking part in a blockade on the M25 on 8 October, while Ana Heyatawin, 58, and Louis McKechnie, 20, were jailed for three months.
The biologist then vowed to stop eating until the government issued a “meaningful statement” to begin insulating more homes.
Speaking from HMP Bronzefield, the activist said: “The window of my cell in the hospital wing is blocked up and there is little natural light, in my previous cell I could see the birds and trees that line the prison fence.
“I have less time to go outside in the prison yard for exercise now. All of this is testing my resolve to continue, but I feel that not eating is the only thing I can do from prison to draw attention to those who will have to make the choice between heating and eating this winter.
“Not standing by while our government commits treason against the people of this country feels like the most important thing I will do in my life.”
On Tuesday, Insulate Britain protesters plan to stage a 24-hour fast outside 10 Downing Street to “highlight the need for urgent action on fuel poverty”, with Ms Smith’s husband, Andy, in attendance.