Insulate Britain are holding a 24-hour fast outside Downing Street in solidarity with a fellow jailed protester who has been moved to a hospital wing after going on a hunger strike.
The eco-activists say prison staff at HMP Bronzefield, Surrey, have become 'increasingly concerned' about the health of Emma Smart.
Ms Smart, 44, a biologist from Weymouth, has refused food for the past 14 days since being jailed for her part in Insulate Britain's road-blocking protests.
Today a handful of supporters gathered at No 10 to start a 24-hour fast, not blocking the road as in previous demonstrations that have outraged motorists.
A spokesperson said: 'Supporters of Insulate Britain will be doing a 24-hour fast outside 10 Downing Street, with Emma, in solidarity with people who will have to make the choice whether to heat or eat in Britain this winter.
'This is the only option now available to her in prison, to highlight the cowardice of in refusing to support the most vulnerable.'
The No 10 fasting is supposed to last 24 hours, which will be a challenge in cold temperatures
The controversial activists left little doubt who they were there for with their placards
Andy Smith and his wife Emma Smart, who was handed a four month prison sentence
It comes as the group yesterday released a statement on behalf of the jailed eco-activist.
Speaking from prison, she 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.
Supporters of Insulate Britain stage a 24 hour fast outside Downing Street, London, in a call for action on fuel poverty
The protests are in solidarity for a hunger striking Insulate Britain group activist who is among those who received prison terms for blocking roads.
Smart was imprisoned with eight other people from Insulate Britain (pictured: An Insulate Britain protest) who were given sentences of between three and six months and were each ordered to pay costs of £5,000
Smart, 44, has been on hunger strike for 14 days since being jailed for her parts in Insulate Britain's disruptive road-blocking protests
Jailed Insulate Britain activist Emma Smart (pictured), who is on hunger strike while in prison, has been taken to hospital, the group say
'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.'
Smart was imprisoned along with eight other Insulate Britain activists who were given sentences of between three and six months after breaching an injunction designed to stop the group's road-blocking protests.
They were also each ordered to pay costs of £5,000. The court ordered that the jailed activists should serve at least half their sentences.
A further group of nine people from Insulate Britain have been summoned to appear at the High Court next month to face a charge of contempt of court.
If found guilty, they could be subject to unlimited fines, seizure of assets and prison sentences of up to two years.
Smart, who was jailed earlier