Defra has today rejected calls from carbon offsetting firms to allow voluntary carbon credits to carry a new government-approved quality mark, but has insisted that the "door is still open" for voluntary credits to enter the new quality assurance scheme as long as the offsetting industry can develop its own robust quality standard.

Announcing the results of a consultation period into the proposed offsetting code of conduct and quality mark, environment secretary Hilary Benn said that the code would initially only cover certified credits that are compliant with UN and EU-backed carbon markets.

"I think it's right that we set a high standard," he said. "It's important that consumers who want to buy carbon offsets with confidence can do just that. When a consumer buys a tonne of carbon with the Government's quality mark, they'll know they're buying a full tonne of carbon."

The new quality mark is now scheduled to be launched by the middle of year and is likely to be carried by a growing number of offsetting providers offering UN-approved Certified Emission Reduction (CERs) credits, including BA, Lastminute.com and offset specialist ClimateCare.

Providers of voluntary carbon credits that have not been certified by the UN or EU had been lobbying for an extension of Code of Practice's criteria to include high quality voluntary credits. They argued that many legitimate carbon reduction projects were unable to gain CER accreditation through the UN's Clean Development Mechanism (CDM) due to the high administration costs associated with certification and the UN's reluctance to approve many small scale projects.

A Defra spokeswoman said the government had not been convinced by the argument that voluntary credits offered sufficient certainty to customers that emission reductions had been achieved. But she added that the "door was still open" for voluntary credits to carry the quality label if the offset industry developed its own standard capable of ensuring credits deliver additional carbon reductions, are properly verified and follow robust accounting processes.

"The challenge to the offsetting industry is clear," said Benn. "To establish a clear, rigorous standard for voluntary projects that deals with the concerns that have been raised. We will support them in developing that standard – and when we have the necessary guarantees, we'll include high-quality voluntary offsets in the Code."

Mike Mason, founder and chairman of offset provider Climate Care, which offers both voluntary credits and CERs, said that the industry could deliver a robust voluntary code of practice within months. "We don’t have to invent anything new we just have to make some changes to some of the existing code of practices to ensure legitimate voluntary projects are covered," he said. "We should be able to offer a robust standard within a matter of months and that will turn the challenge back on the government to approve it."

A spokeswoman for offset provider The CarbonNeutral Company also welcomed the proposals, arguing that a government-approved standard would help discourage disreputable operators. "There has been exponential interest in reducing carbon emissions through offsetting and in the number of organisations setting up to offer carbon offsetting services," she said. "A Code could help ensure that increasing numbers of operators do not have a diluting effect on standards."

Mason also welcomed the launch of the new quality mark, but raised questions over how many customers will be willing to pay for the more expensive certified credits. "CERs are traded in a large liquid market where you get firms asking for 10m credits a year," he explained. "As a result CERs cost around £12 a tonne, compared to £8 for voluntary credits."

In related news, US certification body Green-e last week launched a new quality label designed to offer independent verification for offsets aimed at the consumer market.