Mobile users in the UK who overcharge their phones waste almost £40m a year, and damage the environment and their batteries in the process.

A survey by comparison site MoneySupermarket.com found that most people leave devices to charge for well over eight hours, mainly overnight, when the majority are fully charged in around three hours.

The figures suggest that around 85,000 tonnes of CO2 could be saved if people disconnected their handset as soon as the charging was complete.

The younger generation are most at fault, with 49 per cent admitting to overcharging their mobiles by an average of 7.3 hours every time they plug it in.

Women were the most environmentally conscious, according to the survey, with just 11 per cent admitting to overcharging for an average of 6.8 hours, compared to 15 per cent of men who overcharge for an average of 8.5 hours.

"There have been some great advances in green technology from mobile manufacturers, but all that goes out the window when people are overcharging mobiles, wasting energy and driving up their utility bills," said James Parker, manager of mobiles at MoneySupermarket.com.

"Many people can't imagine going a day without their mobile, and have them almost permanently plugged in. The average phone takes only two to three hours to charge up, and overcharging can actually be damaging to the battery life."

Mobile manufacturers have made great strides in making devices increasingly eco-friendly. Nokia's phones provide messages reminding users to disconnect their chargers, and there are standardised energy efficient chargers under development, as well as a number of green mobile phones and recycling schemes.

Parker recommended that mobile owners should only charge their phone when the battery is completely dead, and for up to four hours only. Those who charge their phones overnight should invest in an electronic plug timer to switch off the charger after a few hours.