COMPLAINTS about companies that claim to help to protect the environment have forced the Advertising Standards Authority (ASA) to consider issuing guidelines on what businesses can say about the practice of carbon offsetting.

The voluntary schemes estimate the carbon emissions generated by companies or individuals and then offer to combat the environmental damage they cause by investing in such projects as planting trees, using energy-efficient lightbulbs and investing in wind farms to soak up, or save, the same amount of gas.

Last week the advertising watchdog told Scottish and Southern Energy that it had failed to substantiate a claim and told it to drop an advertising campaign.

The energy supplier had implied that it would plant trees to absorb the same amount of carbon as that produced when gas is used to heat an average UK household. But some forms of carbon offsetting, particularly the planting of trees, have been criticised. Trees take up to 100 years to use up the amount of carbon they have been planted to offset — and that carbon would return to the atmosphere if they burn down, for example.

A spokesman for the ASA said that its ruling on Scottish and Southern should be seen as a warning that firms needed documentary evidence to back up their claims.

The Committee of Advertising Practice, which puts together codes of practice policed by the ASA, said: “In the last year there have been more and more claims on carbon offsetting It is a growing trend and we have to set an agenda.”

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