Some of Britain's most iconic landmarks, including Buckingham Palace and Big Ben, will be plunged into darkness for an hour on Saturday evening to raise awareness about climate change.
London's Tower Bridge, Blackpool Pier, Edinburgh Castle and Manchester United's Old Trafford football stadium will also take part in the World Wildlife Fund's (WWF) Earth Hour at 8.30pm.
London's South Bank will be the "focal point" for the Earth Hour switch off, WWF said, where visitors can experience candlelit dinners in many of its bars and restaurants.
British tennis champion Andy Murray, a WWF global ambassador, has backed the campaign along with Prime Minister David Cameron and Labour leader Ed Miliband.
More than 700 landmarks around the world will also switch off their lights, including Christ the Redeemer in Brazil, France's Eiffel Tower and more than 30 Unesco World Heritage Sites.
Earth Hour was launched in 2007 in Australia and has since spread around the world.
Last year nine million people in the UK took part, WWF said, as part of a record-breaking chain of 162 countries.