Gordon Brown does not see the environment as important and spent years as Chancellor preventing British domestic action on climate change, the Government's chief environmental adviser says today.

In a remarkably frank interview with The Independent Jonathon Porritt says that during his time at the Treasury Mr Brown did not "get" climate change and saw the environment as "middle class stuff". Since Mr Brown became Prime Minister, Sir Jonathon thinks that the environment "is no more important to him now than it was when he was Chancellor". He criticises the Prime Minister in particular for backing the third runway at Heathrow, which he says was a "ludicrous decision, with no serious intellectual, economic rationale".

Asked why he thought Mr Brown was so keen on a third runway, Sir Jonathon said: "He's got 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. And you can put the evidence about minimal economic benefit of a third runway in front of him – it won't go through the Brown brain." But, he says, over the past 18 months Mr Brown has come to understand the seriousness of the climate change issue specifically, and has taken a number of important steps, such as setting up the new Energy and Climate Change Department under Ed Miliband.

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This has injected a "completely unprecedented level of energy" into the climate debate, Sir Jonathon says. He adds it is ironic to be leaving his post just at the moment when real changes in the system are starting to happen. He retires on Monday after nine years as chairman of the Sustainable Development Commission, the Government's main environmental advisory body.

Sir Jonathon says that while Tony Blair was trying to put the issue of global warming high on the international agenda, Mr Brown "held the levers of power domestically" and failed to deliver appropriate climate policy measures at home.

For Mr Blair to have Mr Brown in such a position was "a very unfortunate combination", he says, and it was "a tragedy" that Mr Blair was unable to match what Sir Jonathon calls his "inspirational" leadership on the climate issue internationally, with action taken in Britain. As a result, the gap between Mr Blair's rhetoric on climate change and the lack of actual measures taken to counter it in Britain became so great that environmentalists began to lose faith in him.

"Environment has never been Gordon Brown's strong suit," he says.

Sir Jonathon is a serious player in green thinking, and for nearly a decade at the head of the SDC has been the official "candid friend" to the Government on green matters, running a body which has tried to get the notion of sustainable development – environmental concern, in other words – embedded in every department and policy.

It has given him a uniquely intimate view of the Labour Government's environmental successes and failures, one of the most obvious of the latter being the lack of measures taken to counter climate change in Britain during the years between 2000 and 2007 when Tony Blair was proclaiming the seriousness of the threat.

"Tony Blair fully understood the risk of climate change, and whatever you may think about him, his commitment on that was completely genuine," he says. But, he goes on, the Blair-Brown partnership was the problem.

"It was a very unfortunate combination, with Tony Blair having the unquestioned leadership role globally, but not having the levers to make things happen domestically. And the sort of informal deal they [Blair and Brown] did, meant that far more of that went to the Treasury."

Although Mr Blair "got" climate change, Mr Brown, at that time, "absolutely didn't," Sir Jonathon says. "I rather suspect – although I'm sure he would deny this completely – that the environment was in many respects seen by Gordon Brown as part of that middle-class stuff going on over there, while we do the serious business of sorting out equity issues and entitlements, and how to make the economy really efficient."

Mr Brown, he says, is a man "of great intellectual strengths and integrity," but if you looked back into his personal history, his mentors, and his view of the world, the environment had not been part of the growth of his ideas. He did not see it as central.

But he had now changed his view on the climate issue not least because he could see its potential impact on areas which had always mattered to him, such as global poverty and Africa.

"I don't think he had thought terribly deeply about it when he became Prime Minister, but to be fair, he's now much better informed, and he's really got his head around these things," he says. "At the recent G8 meeting and the subsequent meeting with China and India he was playing an extremely significant role in the negotiations." Sir Jonathon says he wonders whether, if Mr Brown had to take the Heathrow third runway decision now rather than a year ago, he would still approve it. "He won't go back on it now because he's not that kind of politician. But I have watched how over the last 18 months his leadership on climate change has deepened."

Sir Jonathon is optimistic that the world community can reach a new deal at the Copenhagen climate conference in December to cut carbon emissions and check the progress of global warming before it becomes disastrous, although he thinks that over the coming years the world is in for some "serious shocks" either from climate change or resources such as food and water.

However, he believes they may be what is needed to galvanise action from governments and citizens and will not necessarily be terminal. He says that the dire predictions of James Lovelock, founder of the Gaia theory of the world as a single organism – "that the world is headed into some eco-hell-hole" – are too pessimistic. "Though I say that with some hesitation as I do have a huge amount of respect for that guy."

After he leaves the SDC on Monday he will spend most of his time working for the think-tank he founded with Sarah Parkin, Forum for The Future, and also for the Green Party.

At the forefront of green campaigning

*Sir Jonathon Porritt, 59 is the doyen of Britain's environmentalists, the first green campaigner to become nationally famous. He is a former director of Friends of the Earth.

*He is the son of Arthur Porritt, the New Zealand athlete and surgeon who won the 100 metres bronze medal in the famous "Chariots of Fire" race at the 1924 Olympics. His father also went on to become Lord Porritt and New Zealand's Governor-General.

*Educated at Eton and Oxford, Porritt is in theory doubly-titled – he is "Hon" as the son of a life peer, and also Sir Jonathon Porritt, Bart, having inherited the baronetcy which was also awarded to his father. But he makes little of any of this and is a very different creature from the aristo-greens and celebrity-greens who pop up from time to time on Britain's environmental scene.

*He does not own a car, but has two bikes – one for home in Cheltenham and one for work in London.