The Houses of Parliament, luxury hotels like the Ritz and key businesses have confirmed they will take part in the "Lights Out" campaign from 9:00 pm to 10:00 pm (2000-2100 GMT) Thursday.
The initiative, spearheaded by a local radio station and backed by Mayor Ken Livingstone, comes on the longest day of the year in Britain when the sun is due to set in the British capital at 9:21 pm (2021 GMT).
"Excessive illumination contributes to climate change and you may like to join in, if you are in London tonight, the London Lights Out campaign," said Ben Bradshaw, minister of state for the environment, in parliament.
Also participating are the Canary Wharf financial district and the 175-metre (574-feet) tall BT Tower. The government on Wednesday launched an online trial calculator that enables people to figure out their "carbon footprint," or how much the consumption of energy releases carbon dioxide into the air.