Many firms will have to recalculate their carbon footprint after the government today announced a major crackdown to stop customers automatically rating energy purchased under green energy tariffs as carbon-neutral.

Demand for green energy tariffs has soared in recent years as firms seek to shrink their carbon footprint. However, the environmental benefits of different green tariffs vary enormously, and some corporate customers are likely to be making lower carbon savings than they expect.

For example, some green tariffs simply sell – often at a higher price than standard tariffs – the renewable energy that suppliers already have to produce under the government's Renewables Obligation mechanism. Others rely heavily on carbon offset schemes instead of sourcing the energy from renewable sources.

But environment secretary Hilary Benn said today that the government had changed its guidelines on carbon reporting to stop firms automatically claiming that by signing up to a green tariff they have zero energy-related emissions.

"For the reporting year 2008/09, best practice is expected to be for businesses to use a grid average rate – the average rate of carbon emissions associated with electricity transmitted on the National Grid – unless their supplier can prove the carbon benefits are additional [to what is required from them under the Renewables Obligation]," Benn said.

"This is to reflect the existing evidence that the additional carbon benefit of green tariffs is not transparent."

The changes are likely to have repercussions for a large number of firms that use green tariffs to underpin their claims of carbon neutrality.

It will also crank up pressure on energy suppliers to ensure their green tariffs sell energy sourced from renewable developments that exceed those required under the Renewables Obligation.

To help firms re-evaluate the contribution of green tariffs to their carbon footprints, Benn said he had written to the chief executives of energy suppliers to ask them to provide the clearest possible information about the tariffs' environmental benefits.

Energy watchdog Ofgem is also undertaking a review of the green tariff market and aims to have an independent accreditation scheme in place by the end of the year to make it clearer how different tariffs work.

A spokeswoman for Defra said that the aim of the changes was to provide greater transparency.

"We want to be 100 per cent sure that businesses are clear what they are getting for their money," she said. "It is easy for firms to buy a green tariff and tick that box, but these changes are about challenging them to look more closely at what they are buying."

Juliet Davenport, chief executive of green energy supplier Good Energy, welcomed the news, but urged Defra to base its carbon reporting guidelines on a supplier’s total fuel mix, which suppliers are already obliged to report, and not on the grid average. "This will better reflect whether a supplier is making a serious commitment to reducing carbon or just meeting their current obligation ".