O2 has unveiled a scheme that ranks mobile phones based on how green the handset is.

The O2 Eco Rating scheme, which is being run in conjunction with Forum for the Future, gives each phone a rating from zero to five based on how 'green' the raw materials it is made from are, as well as the environmental effect of the phone's manufacturing process, its packaging, how energy efficient the handset is and also how easy it is to recycle.

According to the mobile network, more than two in five (44 per cent) of its customers would choose a handset based on its environmental credentials.

O2 said the Sony Ericsson Elm was the most environmentally friendly handset with a four of 4.3 out of 5, and closely followed by a number of Nokia and other Sony Ericsson handsets. Meanwhile, the LG Etna was named the least environmentally friendly phone with a Eco rating of just 2.7, and was closely followed by the Palm Pre Plus and the LG Chocolate.

More than 90 per cent of the handsets available on the mobile network have neen rated under the scheme, but while Nokia, Sony Ericsson, LG and Samsung have agreed to take part, Apple, which manufacturers the iPhone - one of the most popular phones on the network, has refused.

According to The Guardian, Apple has declined to reveal why it's not taking part in the scheme.

"We know that sustainability is important for many of our customers and for the first time they will have the whole picture from which to make a full and balanced purchasing decision," Ronan Dunne, chief executive of O2 in the UK, said.

"Consumers are becoming increasingly aware of the social and environmental impact of the technology they buy and we believe this kind of product transparency will help empower them to make greener choices."

James Taplin, from Forum for the Future, said in a blog that the Eco Rating scheme had been developed "in close collaboration with handset manufacturers" and gives UK consumers" the information they need to make an informed choice about the devices they use".