Companies offsetting their greenhouse gas emissions by purchasing carbon credits on the international carbon markets have been warned to verify that their investments are going towards genuine projects to reduce emissions.

An increasing number of companies, including HSBC bank, Marks and Spencer and BritishSkyBroadcasting, are choosing to become “carbon neutral”, meaning they have no negative impact on the climate. This is achieved by reducing a company’s emissions through energy efficiency measures and the use of renewable energy, and then “offsetting” the remainder.