Carbon offsetting Web sites are inconsistent and confusing, with costs varying by up to 540 percent, according to a report.

A survey by Which? Money found huge variations in how such offsetting schemes calculate people's "carbon footprint" - and how much they charge.

A range of activities affect the carbon footprint of individuals, from driving to work and flying overseas to heating and lighting our homes and buying food from non-local sources; the higher the "food miles" before it reaches the plate, the greater the environmental impact.

But there are now hundreds of carbon offsetting schemes that aim to help individuals and businesses reduce their CO2 emissions by offsetting, reducing or displacing the CO2 in another place, typically where it is more economical to do so.

They use the money to invest in renewable energy, energy efficiency and reforestation projects around the globe.

Which Money?, a consumer magazine, compared the Web sites of 13 UK-based carbon offset companies, and found that emissions based on the same example varied from 1.15 tonnes with Carbon Footprint to 7.1 tonnes with the Carbon Neutral Company.

The government calculator (www.direct.gov.uk/ActonCO2) gave an emissions figure of 4.31 tonnes for a test home.

To add to the confusion, the cost of offsetting a tonne of carbon dioxide varied from 7 pounds with Carbon Offsets to almost 23 pounds per tonne with Carbon Responsible.

The total cost to offset the carbon produced by the test home ranged from 25 pounds to almost 160 pounds - 540 percent more.

Which? Money also uncovered a lack of transparency.

Few of the carbon offset companies gave sufficient information on the administration fees involved or proportion of the money that reaches carbon offset projects.

Climate Care was the most transparent, with Blue Ventures Carbon Offset, PURE and the World Land Trust also highly rated.

The government plans to introduce a voluntary code of practice this spring to cover carbon offsetting projects that comply with the Kyoto treaty on climate change, but it will not apply to non Kyoto-compliant schemes.

Martyn Hocking, editor of Which? Money, said that should aid transparency.

"Carbon offsetting schemes offer to ease your conscience, but choosing which company to use can be very confusing as there's no consistency in how they calculate your 'carbon footprint' or how much they charge," he said.

"The new code of practice will help indicate which schemes meet standards of transparency and quality.

"As it doesn't cover all schemes, we'd like to see the industry develop its own code of conduct so that people can donate with confidence and know that their payment is being used for a verifiable project."

Only 7 percent of 2,645 Which? members surveyed have used carbon offsetting schemes, but 68 percent of those who have not would consider doing so in the future.

The survey of carbon offsetting Web sites was based on a typical example of a couple living in a two-bedroom semi-detached home in west London, spending 500 pounds per year on gas and 300 pounds per year on electricity, driving a petrol-engined Ford Focus 8,000 miles a year and taking one return flight each a year from London Heathrow to Barcelona.