The chairman of the UK Sustainable Development Commission has warned that power companies may one day face the same legislative backlash as big tobacco companies.

Speaking at a debate on environmental policy at the Liberal Democrat's conference in Bournemouth yesterday, former director of Friends of the Earth, Jonathon Porritt, said the decision last week to acquit protestors at the Kingsnorth power plant in Kent was a sign that power companies could find themselves subject to increasingly tight legislation.

"This has sent tremors through the power companies, tremors through government, and earthquakes through the Daily Mail, which is totally horrified by the idea that civilisation has collapsed," he said. "For a long time energy companies have been warned that a situation will emerge in which they become similar to the tobacco companies, which for many years sought to deny that they had any corporate culpability for the damage done to personal health."

Campaigners in the UK and the US have long suggested that big energy companies could be liable for prosecution over their contribution to climate change, Porritt said, and the Kingsnorth decision made such a scenario look increasingly likely. "All energy companies wave that away as if that idea was completely ridiculous, but I can tell you now that they are probably thinking about it a lot harder now then they were," he said.

Six Greenpeace activists were charged with causing £30,000 of damage to the Kingsnorth power plant in Kent last October. But last week they were acquitted of criminal damage with the jury agreeing by a 10 - 2 majority that "emergency action" could be used to stop damage to the environment.

Greenpeace argued that under the Criminal Damages Act of 1971, the campaigners had a lawful excuse to cause the damage as they were trying to prevent even greater damage being caused by the contribution the plant would make to climate change.

Porritt said that he was "astonished" by the Kingsnorth verdict, and that it paved the way for environmental legislation that could have a real impact on energy companies in the future. "This is a really big decision and it will have woken up a lot of people to the potential consequences of climate change in the not-too-distant future," he said. "We are not talking hundreds of years here, we are talking decades."

Porritt was appointed by the then Prime Minister Tony Blair as chairman of the UK Sustainable Development Commission in July 2000. The organisation is recognised as the Government's main source of independent advice on sustainability issues.

Also taking part in the environmental policy debate was Liberal Democrat MEP Chris Davis, spokesman for environment and public health, who claimed that all politicians should be prepared to take direct action for issues they believe in.

"If we are on the losing side of something we feel passionate about, and we are prepared to take the risk and consequences of a judgment against us then I don't have a problem taking part in an act of non-violent direct action," he said. "If Gordon Brown decides to go ahead - and it looks like it is down to him now - with Kingsnorth without CSS (carbon capture storage systems) then I will sit down in front of the bulldozers with other protestors and take what comes to me."

Davies said he is taking the lead on the development of carbon capture and storage technology in the European Parliament, and in three weeks' time will meet with the Environment Committee to vote on an amendment which, if successful, he claims "will save HM Treasury hundreds of millions of pounds necessary to support carbon sequestration projects".