World in the dark over climate change
The lights will go out in towns and cities around the world this month as part of a global initiative to highlight climate change.
WWF-UK head of campaigns, Colin Butfield, said: "This year's Earth Hour hopes to attract some one billion people.
"By signing up to switch off their lights, individuals, businesses, towns and cities can join a global phenomenon and show world leaders that we care about climate change."
This year's event, hailed as the biggest yet, will ripple across the earth as communities dim the lights in 25 time zones, including on some of the world's most iconic landmarks.
These include San Francisco's Golden Gate Bridge, the Las Vegas strip, Table Mountain in Cape Town, South Africa and Dubai's new Burj Khalifa tower - the world's tallest building.
They will be joined in the UK by Piccadilly Circus, the Royal Albert Hall and the London Eye in London, Manchester's Trafford Centre, Wales Millennium Stadium and Portsmouth's Spinnaker Tower.
Countries and regions taking part for the first time include Madagascar, Nepal, Saudi Arabia, Mongolia, Cambodia, Czech Republic, Paraguay, Ecuador and the US Commonwealth of the Northern Marina Islands in the Pacific Ocean.
The Earth Hour event first took place in Sydney in 2007 with some two million people switching off their lights.
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