Set to coincide with the longest day of the year, Lights Out London will pull the plug on some of the city's tourist hotspots in order to raise awareness of global warming and energy saving issues.
Amongst the buildings set to be plunged in darkness are St. Paul's Cathedral and Buckingham palace, as well as the houses of parliament in Westminster.
And Picadilly's iconic advertising will be turned off for the first time since the end of WWII.
Private homes are also being encouraged to turn their lights off at 9pm in order to save energy.
Comparison website moneysupermarket.com estimates that as much as 900,000 kWh of energy could be saved thanks to the measure.
This could be as much as 36 million kWh of energy if all non-essential electrical appliances are also switched off across UK homes.
"In just one hour, a single household can save about £1.15 by switching off a range of appliances, or £30 million across the UK," said Paul Schofield, head of utilities at moneysupermarket.com.
"While this campaign will help us think about energy consumption, it would be fruitless financially if we were languishing on an expensive electricity tariff."
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