The prime minister has said he wants the new coalition administration to be "the greenest government ever". David Cameron was speaking to civil servants at the Department of Energy and Climate Change on his tour around Whitehall to introduce the new government.

Flanked by Liberal Democrat Chris Huhne who will run the department, Cameron reassured civil servants that the environment was a top priority for him. "There is a fourth minister in this department who cares passionately about this agenda and that is me, the prime minister, right. I mean that from the bottom of my heart."

In his introduction, Huhne said, "Climate change is in my view, our view, the greatest challenge facing mankind," adding that he wanted to go "further and faster than ever before".

Cameron used the four-minute speech to formally announce the coalition's pledge to cut carbon emissions by 10% in the first 12 months. He said this would be helped by a new innovation to publish the energy being used by government departments in real time. This is transparency in action to drive forward the cuts in carbon emissions. It's a new way of doing climate change, it's a new way of doing politics, it's in the spirit of our age and I think it will make a very big difference," he said. Cameron could not resist a side-swipe at the out-going Labour government. "Something I hope we can do is try and improve the record in terms of tenure of ministers," he said - a reference to the average term for Labour energy ministers of just over year. And he framed the coalition's plan to introduce five-year fixed term parliaments as a benefit for dealing with environmental issues. "Nowhere are long-term decisions more needed than actually in the fields of energy and climate change and environment."

The prime minister picked out three areas from Huhne's in tray that would need special attention - the green economy, climate change and energy security. "We've got a real opportunity to drive the green economy to have green jobs, green jobs and make sure we have our share of the industries of the future," he said.

Deborah Doane, director of the World Development Movement welcomed the commitment to a 10% reduction in carbon emissions from central government. But she added, "History will judge this government on its green credentials by its policies to cut the UK's emissions dramatically and getting a fair international climate deal, not by turning off its lights at night. It doesn't take the scale of the problem seriously, any suggestion that blue and yellow means green government are premature because there are so many unanswered questions about the policies."

She expressed concern that the coalition agreement and the Conservative manifesto did not say anything about post-Copenhagen climate negotiations. "This is worrying as the whole Conservative manifesto also ignored its approach to international agreements," she said.

Will Day the chair of the Sustainable Development Commission an official government watchdog said: "This is an excellent first step, which has the potential pay dividends in terms of saving public money through energy efficiency, and contributing towards a sustainable, low-carbon economy for the future."