One in eight Brits are leaving their mobiles on charge even when the battery is full, which amounts to 85,000 tonnes of excess CO2 being produced every year, says Moneysupermarket.com.
Research by the comparison website revealed that teenagers are the worst offenders with nearly half of all under 20s admitting to charging their handset for an average of 7.3 hours a time.
Moneysupermarket.com said overcharging mobiles adds £40m to electricity bills each year.
James Parker, manager of mobiles at moneysupermarket.com, said: "There have been some great advances in green technology from mobile manufacturers recently but all that goes out the window when people are overcharging mobiles - wasting energy and driving up their utility bills."
Parker urged consumers to only charge their handsets for up to four hours at a time and invest in an eco-charger that stops drawing power once the battery is full.
"The average phone only takes about 2-3 hours to charge up and overcharging can actually be damaging to the battery life. People should remember to unplug the phone charger when they are done; it will not only be good for the phone's battery life, but good for their wallet too."
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See also: Mobile consumers want greener phones