A new website allows online shoppers in the UK to progressively offset their personal carbon footprint and earn cash back without paying any extra.

Users accessing normal online shopping stores via the Smartly Green site pay no more to shop, but the company gets a commission from the retailers.

Half of the funds raised is credited to the user's loyalty scheme account, of which 80 per cent is donated to environmental charity Pure, the Clean Planet Trust.

The remainder is held in the member's account, available as cash to spend or donate once they have offset the chosen annual carbon profile target.

At that time the breakdown is reversed so that 20 per cent of the loyalty commission is donated to Pure and 80 per cent returned as cash.

The site has already set up arrangements with major online shopping retailers including Tesco, Boots, John Lewis, Apple, Asda, Carphone Warehouse, Avon and Adili.

"Join me by cutting down your household's carbon footprint, and offset what remains by donating to Pure. It's the most sophisticated solution to fighting climate change," said Stephen Fry, actor and patron of Pure.

Members of the scheme can monitor their progress towards their chosen personal carbon offset target as they shop, and Smartly Green has calculated that users can expect to offset about half a tonne of carbon for every £500 spent online.

The average annual household carbon footprint is about 10 tonnes, but can range from two to more than 20 tonnes depending on lifestyle.

Funds paid to Pure are directed to renewable energy and efficiency projects around the world which adhere strictly to the high quality international standards of the Kyoto Protocol.

"The UK government welcomes the launch of Pure's carbon offsetting scheme," said Ian Pearson, minister for Climate Change and Environment.

"The scheme offers credits that have been generated from emissions reductions that are regulated, verified and for which there is a clear audit trail."

Online retail sales this Christmas are expected to top £10bn in the UK, according to research firm YouGov.

Smartly Green reckons that if 10 per cent of these sales are directed via its site, UK shoppers could collectively offset an estimated 1.7 million tonnes of carbon, covering the average annual footprint of about 170,000 households.