The UK is unlikely to meet its 2010 target of reducing carbon dioxide emissions by 20%, the government's chief scientific adviser has admitted.
Sir David is the UK government's chief scientific adviser
Sir David King told the BBC the target was perhaps a "bit optimistic" but said the government had not given up and long-term plans were in place.
The "green light" should be given for more nuclear reactors, he added.
Environmental groups accused Prime Minister Tony Blair of backtracking on the issue of setting targets.
Sir David told the BBC's Sunday AM programme that the 2010 target on reducing emissions was a "very tough target to hit at the moment".
He admitted the UK could miss it but said one reason was that long-term plans took time to pay off.
"The longer term targets are actually the critical ones. These things like building a new power station take many, many years to come through.
"I think perhaps we were being a bit optimistic, but the government has not given up on its target for 2010."
Sir David said Mr Blair should "give the green light" to a new generation of nuclear reactors.
Nuclear power met almost a quarter of Britain's electricity needs in recent years but that will fall to just 4% by 2020 if reactors were not replaced.
"All of that is coming from a CO2-free source. I think we need every tool in the bag to tackle this problem," he said.
Mr Blair faces stiff opposition from green groups and some in his Labour Party if he sanctions new reactors.
Environment minister Margaret Beckett said there was "nothing extra" nuclear power could do to help meet the Government's target to reduce CO2 by 2010.
Speaking on the BBC's Politics Show, she said: "There's just no way you could get new nuclear power stations in time to contribute to that."
She denied she was anti-nuclear but said there were "lots of concerns" about its use.
Earlier this month, the prime minister caused fury by suggesting that a "child-of-Kyoto" agreement, with firm targets for reducing greenhouse gas emissions, could be tricky.
"People fear some external force is going to impose some internal target on you which is going to restrict your economic growth," he said.
"I think in the world after 2012 we need to find a better, more sensitive set of mechanisms to deal with this problem."
Greenpeace director Stephen Tindale, a former environment adviser to Labour, told Sunday AM that Mr Blair realised he could not persuade United States President George Bush over the issue.
President Bush refused to ratify the Kyoto treaty when 141 countries signed up to it this year.
"I think Tony Blair has realised that he is not going to shift George Bush and he is therefore trying to move the rest of the world to George Bush's position, which is a disaster," Mr Tindale said.
He insisted that it would be impossible to "get a handle" on the future without emissions targets.
Sir David said he believed Mr Blair's comments on targets had been misunderstood and that Mr Blair had been talking about involving China, India, Brazil, Mexico and South Africa in discussions.
"I believe what he was discussing was... how do we extend that to include these five countries? Of course we're also concerned about the United States' position.
"The US emits 25% of the world's carbon dioxide. How do we bring them on board?"
World leaders are due to discuss the issue at a United Nations Framework Convention on Climate Change meeting in Montreal, Canada, from 28 November.