Office workers who leave their computers on are wasting £120m a year, according to research by the Energy Saving Trust.
Some 75 per cent of workers claim to be environmentally aware, but over a third do not turn off their machines overnight.
The Trust said that this results in a million tons of carbon dioxide being pumped into the atmosphere every year. It also estimates that home computer users waste £41m a year and pump out an extra 220,000 tons of carbon dioxide as a result.
"People think they are saving money using standby, but in fact a PC or TV uses 90 per cent of the energy that it would if fully turned on," said a spokesman for the Trust.
Environment group Friends of the Earth has called for regulations ensuring that manufacturers design electronic devices with standby options that use far less energy.
Climate change campaigner Martyn Williams said: "This is a classic case of the government leaving it to the individual, but that will never solve the problem.
"It would be far easier if there were rules to insist that something is done. Manufacturers could easily adapt their designs or we could use special plugs to detect if devices are on standby."
David Pincott, managing director of marketing agency Pirate Communications, added: "We do not leave PCs on standby in our office, nor do I at home, partly as a result of a lecture I was given by a fire and safety officer.
"The problem is the time it takes to power up a PC. We used to have it with TVs and now we have the same problem with PCs, so its not surprising people use standby."
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