The 'Lights Out London' campaign is supported by the Greater London Authority, EDF Energy, charity WWF and London freesheet Metro.
It's aiming to get 3m people involved and save around 750MWh of electricity, enough to run 3,000 TVs for a year, through the switch off that will take place from 9pm to 10pm on June 21.
There is a dedicated website at lightsoutlondon.net to explain the initiative and how people can get involved.
Bob Hoad, community relations director and head of corporate social responsibility at Capital owner GCap Media, said: "We hold a unique place in the community when talking to thousands of Londoners every day and therefore play an important part in bringing these listeners together for them to make a difference."
The campaign will be backed by celebrities including Jamelia and Sophie Ellis-Bextor, famous landmarks and businesses.
Ken Livingstone, Mayor of London, said: "At 9pm on June 21, I will be making sure that none of City Hall's non-essential lights and appliances are on."
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