Cameron rues loss of his green standard-bearer

David Cameron's attempts to boost the green credentials of the Tories have been dealt another blow after one of his party's most senior environmentalists announced he is to quit the Commons at the next election.

John Gummer, who had advised the Tory leader on green issues, said he wanted to take up a full-time role fighting for tougher anti-climate change measures after the "deeply disappointing" failure of the recent summit in Copenhagen to reach a legally binding agreement. He said his new role would not leave him enough time to perform his duties as a constituency MP.

Mr Cameron has already been struggling to silence those in his party who remains sceptical about man-made climate change. Mr Gummer co-chaired the Tory party's "quality of life" policy group, which advised Mr Cameron on creating a green economy for Britain.

Its other chairman, the Tory candidate Zac Goldsmith, has been hit by controversy over his tax status. In an interview with The Independent, Mr Gummer issued a plea to the climate change sceptics in his party to rally behind Mr Cameron on the issue. "They should recognise that we are very lucky to have that kind of leadership," he said.

The surprise departure of the Suffolk Coastal MP will frustrate Mr Cameron, who was informed of Mr Gummer's decision before Christmas. Rumours had circulated in Westminster that Mr Cameron was preparing to offer the former environment secretary a role in his government should he win the next election. However, Mr Gummer said the offer of a frontbench job would not have changed his mind. "In a sense, I've done that and I'm aware of the limitations," he said. "I think our front bench is extremely good. I have done a lot with them and will continue to do so. But I feel that somebody has got to be able to talk to the Brazilians, Americans or whoever, in a way that I can do in a direct and non-governmental manner."

Critics of Mr Cameron's pro-environmental stance should try to understand the Conservative leader's position, he added. "If we are wrong on climate change, nothing harmful happens. If we are right, but we follow those who disagree with us, then disaster looms," he said. "We cannot go on living with nine billion people on the planet wasting our resources in this way."