THE MET Office has become embroiled in the outcry over manipulation of climate change data, after it was accused of failing to release details of the role played by its top climate change scientist in a report which has been heavily criticised.

Professor John Mitchell, director of climate science at the Exeter-based centre, was involved in a report produced by the UN Intergovernmental Panel on Climate Change (IPCC). The report, which won a Nobel Prize in 2007, was lambasted by climate change sceptics, who said historic data had been fudged to back-up a theory that temperatures had steadily increased over the 20th century, after being either level or in decline for hundreds of years. It grabbed headlines for claiming Earth is the hottest it has been in 1,300 years.

Critics said the so-called "hockey stick" graph minimised the "medieval warm period" of about 1000AD. Some believe the Earth would have been as warm as it is today during that time.

Climate change sceptics have gathered momentum over the past month, after leaked e-mails between scientists at the University of East Anglia appeared to show information was being manipulated to back up their theories.

Now, Professor Mitchell is in the spotlight over his approval of the final IPCC document, which swept aside concerns over the climatic peak of the medieval warm period. The Met Office has refused to release documents under the Freedom of Information Act (FoI) which would reveal his working papers and correspondence with colleagues. The refusal has been backed by the Ministry of Defence, of which the Met Office is part. Yesterday, a spokesman for the climate change organisation said all relevant documents relating to Professor Mitchell's role were already publicly available.

The applications came from David Holland, an electrical engineer who was among those to successfully secure an Information Commissioner ruling that East Anglia's scientists had acted unlawfully in refusing FoI requests.

Initially, the Met Office claimed Professor Mitchell's records had been deleted. Then, when they admitted their existence, they said his role had nothing to do with the Met Office. They later conceded that the Met Office had paid him for the work – but rejected disclosure on the grounds that it would "inhibit the free and frank provision" of "advice or views" and that it would "prejudice Britain's relationship with an international organisation".

Benny Peiser, director of the Global Warming Policy Foundation, said the affair further undermined the credibility of the IPCC and those associated with it.

"It's of critical importance that data such as this should be open," he said. "More importantly, the questions being raised about the hockey stick mean that we may have to reassess the climate history of the past 2,000 years."

But Met Office spokesman Dave Britton said the FoI requests had been appropriately dealt with and that all documents relating to the process are publicly available on the IPCC website.

He said Professor Mitchell's role in the report was to co-ordinate, and ensure all views were taken into account.