The Carbon Trust has hit back at criticism that it is having a limited impact and not proving cost effective in the wake of a National Audit Office report into its progress.

The report, published last week, found that the Carbon Trust was on track to meet its target of delivering 4.4m tonnes of carbon savings by 2010 and argued that it offered "value for money", having helped deliver businesses up to £655m in cost savings based on investments of less than half that value. It also found that eight out of 10 organisations that worked with the Trust were happy with the service they received.

However, the report also found that only 12 per cent of large businesses with energy bills over £50,000 a year had worked with the Carbon Trust, and when an organisation had worked with firms, less than 40 per cent of its recommendations have been implemented.

The findings prompted criticism that the Carbon Trust was failing to have a sufficient impact. NAO chief Sir John Bourn praised the organisation's achievements but warned that its achievements were small "in view of the scale of the challenge ahead".

Speaking to The Times, Alex Lambie, head of greenhelpline.com, said that at an annual cost of £103m, the Carbon Trust's achievements "are worryingly small and symptomatic of the Government's persistently disjointed and misguided approach to tackling climate change".

But Harry Morrison, senior strategy manager at the Carbon Trust, said that the organisation was pleased with the report, adding that while it was committed to improving further, much of the criticism failed to appreciate the realities of low-carbon investments.

"We think the fact that 40 per cent of our recommendations are implemented is a good figure and we wouldn't expect it to be much higher at this stage," Morrison said. "There are often barriers to investment, in the form of planning and supply chain issues, for example, and some of the responses [to the report] haven’t understood the range of issues businesses have to grapple with."

The NAO report endorsed this view, claiming that the main barriers for firms implementing the Carbon Trust's recommendations were "competing investment priorities and difficulties… securing senior management commitment".

Morrison added that the Trust expected the proportion of its recommendations being implemented would increase over time because a number of its suggestions to firms are large infrastructure projects that can take up to five years to authorise and implement.

Morrison also argued that the organisation was developing new services to help drive up adoption rates further. "One of the main problems is getting senior management buy-in, so we are investing in raising awareness among that constituency of the value of low-carbon investments and are making progress there," he said. "We have also set up systems whereby when one of our consultants recommends a capital intensive project, we then send in a specialist in that area who can help put in the right specs and make the business case."