Forcing companies to publish their carbon emissions – a pre-election promise by both coalition partners – could cost British business up to £6bn over the next decade, the government has claimed in an impact assessment study. But the calculation has been questioned in a report by independent consultants working for the Aldersgate Group, which represents big green-leaning companies such as Microsoft, BT, PepsiCo and Marks and Spencer. The group fears that some ministers and officials will use the environment department’s upper figure – which they believe is far too high – to justify dropping the plans later in the autumn. The department is under pressure from the Treasury and from some business lobbying groups to abandon the policy because of its potential costs on corporate Britain. Mandatory reporting has won a groundswell of parliamentary support, with 137 MPs having signed an early day motion on the issue. The report, commissioned by Aldersgate and three charities – WWF, Christian Aid and the Co-operative Group – says the Department for Environment, Food and Rural Affairs (Defra) appears to have overestimated the total costs of mandatory carbon reporting for large companies by up to £4.6bn and underestimated the benefits by £980m. “We believe that as both the Conservatives and Liberal Democrats pushed for mandatory carbon reporting in opposition, the government needs to commit to full MCR as a way of reducing carbon emissions and backing moves towards a greener economy,” it said. Defra did not dispute the research, which was carried out by Adelphi, an independent consultancy, on behalf of the groups. Instead it said it welcomed all contributions to the debate ahead of the final decision by Caroline Spelman, environment secretary, expected in October. “We asked for comments on our initial impact assessment as part of the greenhouse gas reporting consultation,” a spokesman said. “The work by Adelphi will be considered alongside all other responses to the consultation.” Ms Spelman’s options include keeping reporting voluntary, as it is now. Compulsory measures could mean enforcing reporting for just the listed sector; the 24,000 largest companies; or only the biggest consumers of energy. Defra has produced a range of potential costs and benefits depending on which of the four options is used: applying mandatory reporting to all large companies would produce the biggest potential cost of £6bn. The WWF said: “We are concerned that as the [study] was based on questionable figures, that major benefits to society and the environment will be missed in the final decision because they cannot be quantified as a benefit to UK business.” Many companies already publish details of their carbon emissions, but the practice is not universal or consistent. A recent survey of listed companies by Deloitte found most failed to comply with the government’s voluntary guidance on reporting carbon footprints. Companies already face carbon reporting commitments under the European Union emissions trading scheme and Britain’s carbon commitment. The new system would go further by including all greenhouse gases and not just carbon dioxide. Paul Brannen, head of advocacy at Christian Aid, said British companies were responsible for a large share of global emissions and climate change. “People living in poverty around the world need the government to introduce a robust version of mandatory reporting to give companies and investors all the information and incentives they need to clean up their acts.”