Sir Jonathon criticised the PM's backing for a third Heathrow runway
Gordon Brown's outgoing adviser on sustainable development has accused him of "hindering" work on climate change.
Sir Jonathon Porritt told the Independent the PM did not find the environment any more important now than when he was chancellor.
Sir Jonathon also said Business Secretary Lord Mandelson had to "change his ways" on environmental issues.
Sir Jonathon was appointed Sustainable Development Commission chairman by Tony Blair's government in 2000.
Sir Jonathon said Mr Brown's support for a third runway at Heathrow Airport was a "ludicrous decision, with no serious intellectual, economic rationale".
He said the Prime Minister had "some incredibly fixed ideas about some of these things".
"He genuinely feels that a successful competitive economy of the future has to be growing its aviation business in order to make UK plc more productive, and so on," Sir Jonathon said.
Sir Jonathon has also highlighted Lord Mandelson's influence in government, but added the environmental agenda had not been his "strong suit".
The former UK director of Friends of the Earth, who will step down from his advisory role this weekend, said: "I think the reality is that there are two big things for the government.
"It has got to make the Low Carbon Transition Plan stick; it's got a lot riding on it and it's hugely significant so the government has got to settle down and get implementation."
The other element was developing green industry and technological breakthroughs, he said.
"It's starting to come right, but on the same day Ed Miliband [energy secretary] launched the transition plan, the largest wind manufacturer in the UK announced it was closing," he said.
"We've not got a genuine industrial economic strategy yet and it's absolutely fundamental.
"It's a priority for Lord Mandelson, who has become an immensely influential person in government.
"This whole agenda has not been his strong suit and he needs to demonstrate he can change his ways as the world has changed around him."
Over his nine years at the SDC, Sir Jonathon said times had changed.
"It's taken an incredibly long time to persuade ministers that you can't exhort the whole of the rest of the country to start living more sustainably if you don't demonstrate it in your own back yard," he said.
But, he conceded that the creation of the new Department of Energy and Climate Change last year showed the government has stepped up.
Sir Jonathon plans to continue working with Forum for the Future, a sustainable development organisation, and will campaign on issues including the erosion of human rights in Britain.