It is the first time they will have been off since 1949. They were last turned out in 1939, originally as part of the London blackout during the Second World War and were not switched back on again for 10 years.
Capital's campaign aims to convince millions of Londoners to show their support for energy saving by turning out their lights between 9 and 10pm tomorrow.
In addition to Piccadilly Circus, several other London landmarks will join in, including Buckingham Place, Canary Wharf and The Savoy.
A Capital 95.8 spokesperson said: "We are delighted with the number of businesses and individuals who have signed up, everyone can make a difference if we all work together. London is a city watched by the rest of the world and with the high calibre of supporters on board we can send a really positive message across the world that together we can make a difference."
During the lights-off hour Capital will broadcast a special show, reporting from events across the city and hosting a live gig at its studios with Mutya Buena and Ross Copperman.
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