The ministry overturned a ruling in January by the Danish committee on scientific dishonesty (DCSD), part of the Danish Research Agency, that Mr Lomborg's book The Skeptical Environmentalist was "clearly contrary to the standards of good scientific practice".

Mr Lomborg hailed yesterday's decision as "brilliant". It provided confirmation that freedom of speech extended to the environmental debate, he said.

In its report, the ministry criticised the committee for failing to provide evidence either that Mr Lomborg had been biased in his selection of data or that his methodology had been dubious.

It also said the committee's judgment had used "condescending and emotional" language. And it was a "clear mistake" that the committee had failed to give Mr Lomborg an opportunity to defend himself before publishing its judgment.

This scathing assessment of the DCSD ruling meant that critics would have to find solid arguments to attack his work, rather than rely on mud-slinging, Mr Lomborg said. The ministry's report, which examined only the procedural aspects of Mr Lomborg's treatment by the DCSD, is unlikely to halt the controversy over Mr Lomborg's arguments.

The DCSD was widely criticised in January because it relied on published criticisms of Mr Lomborg's work instead of conducting an independent analysis. Nature, the scientific journal, said the episode "leaves everyone little wiser and the water surrounding Lomborg even muddier".

The DCSD's failure to undertake its own analysis of Mr Lomborg's work also disappointed some of his critics. Christian Ege, director of the Danish Ecological Council, which has published a detailed critique of Mr Lomborg's work, said he was not surprised that the DCSD ruling had been overturned.

Mr Lomborg accused his critics of being motivated by a desire to stop his appointment as director of the Environmental Assessment Institute, established by the centre-right government shortly after it came to power two years ago.

The institute, which has received heavy criticism in recycling-friendly Denmark for claiming it made more sense to incinerate waste aluminium cans than recycle them, has suffered a wave of defections from its supervisory board.

Five members of the seven-strong team have resigned, two for personal reasons and three in protest at Mr Lomborg's plans to host an international conference - Copenhagen Consensus - next spring.

The board members, mostly academics, say Mr Lomborg is overstretching his budget and exceeding his remit by inviting experts and submissions on subjects as varied as financial instability, corrupt governance and infectious diseases.

Anders Fogh Rasmussen, the Danish premier who has defended Mr Lomborg, criticised the board defections earlier this month, while opposition politicians said this latest crisis was another example of Mr Lomborg's lack of credibility.