Two senior bishops are urging people to cut back on carbon for Lent instead of the conventional chocolate or alcohol.
The Bishops of London and Liverpool, Dr Richard Chartres and James Jones, are launching the Carbon Fast at Trafalgar Square with aid agency Tearfund.
They hope to encourage people to reduce their carbon footprint for 40 days.
The scheme aims to raise awareness of global warming to help protect poor communities around the world who are already affected by climate change.
The “fast” involves a simple energy saving action each day, including avoiding plastic bags, insulating the hot water tank and checking the house for draughts.
Bishop Jones, who is vice president of Tearfund, said: “It is the poor who are already suffering the effects of climate change.
“To carry on regardless of their plight is to fly in the face of Christian teaching.”
One Tearfund employee will camp outside the charity’s offices in Teddington for a week in an attempt to reduce his emissions to that of an average Malawian farmer.
Dr Chartres called for “individual and collective action”.