Nick Clegg will warn Tories not to put Britain's recovery at risk by rowing back from environmental commitments as Government advisers said emissions targets should not be watered down.
The Committee on Climate Change (CCC) said there was presently "no legal or economic basis" to amend the UK's legally binding greenhouse gas reduction totals for the period 2023 to 2027.
In an intervention likely to reignite coalition conflict on the issue, the panel of experts said sticking to the target was the "minimum" the UK needed to contribute to global action.
Chancellor George Osborne has warned the UK risks pricing itself out of international markets if it gets "out there in front of the rest of the world" and must keep green measures under review.
He backs a future energy strategy which could see the UK construct up to 40 new gas plants - putting it at risk of breaching the existing targets.
Campaigners fear the Chancellor will seek to use a review of the "fourth carbon budget" scheduled for next year to relax the requirements, blaming the failure of the EU to agree sufficiently tough targets.
But the CCC concluded that the UK was "not acting alone" as other countries had ambitious commitments and that the present target was the " minimum UK contribution to required global action".
Loosening the target " would be bad for business confidence and undermine the UK's credibility in current negotiations over EU ambition", it cautioned.
Climate change is one of the most intense flashpoints of disagreement within the coalition, pitting Lib Dem Energy Secretary Ed Davey against Mr Osborne on the targets and a number of other fronts.
Environmental levies have also become a political hot potato as party leaders battle over how to reduce spiralling domestic energy bills, with Prime Minister David Cameron pledging to scale them back.
Mr Clegg, who has pledged to fight any "hasty, ill thought-through change" in Mr Osborne's Autumn Statement, will use a speech to claim only his party remains committed to the green agenda.
"The same Conservative and Labour politicians who used to shout at one another across the Despatch Box: 'you don't care about the environment, we're the greenest' now turn the accusation on its head: 'you care too much about the environment, you're the greenest'," he will say.
"The green consensus across the political parties is, I'm afraid, falling away.
"And at the worst possible time. Conventional wisdom tells us that the environment must now go on the backburner while we prioritise our economic recovery - but I believe the opposite is true. If there was ever a time to sharpen our focus on our green commitments, it's now."
Delivering the Green Alliance Leadership Lecture in central London, he will add: " Our view is simple: the Government made a commitment to the environment, and we must now stay the course.
"We stuck to our guns on the economic strategy and deficit reduction, despite endless calls to abandon it. We did that because it is right for the generations that will follow us.
"And in exactly the same way, for exactly the same reason, we must hold our nerve on the environment too.
"China, India, America, Germany, Brazil - the race is on with our competitors for green global investment.
"And if we want to keep up, our green industries need maximum political support.
"My coalition partners talk a lot about winning the global race: well this is one area where we are in pole position and it would be a huge mistake to take our foot off the pedal now: economic myopia of the worst kind."
In its assessment - ahead of a detailed report next month - the CCC found " no significant change in the climate science, international and EU circumstances" on which the budget was set.
"Therefore, in these regards, there is no legal or economic basis to change the budget at this time," the independent panel of experts set up to advise the Government concluded.
Among its considerations was the recent Intergovernmental Panel on Climate Change's (IPCC) report that showed global warming was "unequivocally" happening and 95% certain to be caused by human activity.
The CCC said temperature rises of four degrees were " likely" on current emissions levels, the "significant cuts" were required and "that emissions cuts to meet the fourth carbon budget are a minimum UK contribution to required global action".
"The UK is not acting alone: many other countries around the world have made ambitious commitments and are putting in place approaches to reduce emissions," it added - pointing out that the fourth carbon budget was " at the low end of the range of ambition currently being discussed for EU emissions reductions through the 2020s" and may need to go further still.
"If the UK Government is successful in achieving its objectives for EU ambition, a tightening of the budget would be needed."
Chairman, Tory peer Lord Deben, urged the Government to make a swift announcement of its intentions - or risk getting left behind in the race to attract low-carbon investment and jobs.
"A protracted process would exacerbate current uncertainties about its commitment to supporting investment in low-carbon technologies," he said.
Friends of the Earth executive director Andy Atkins said: "The advice is crystal clear: there are no grounds for weakening the fourth carbon budget; in fact it should be made stronger.
"With crucial international talks on climate change kicking off in Poland next week, Ministers could give them a welcome boost by making it clear that the UK is determined to meet its targets for cutting carbon pollution."
Greenpeace policy director Doug Parr said: "Other countries are getting ahead in the global green race with low-carbon solutions.
"But here in the UK, the Treasury persists with its scientifically illiterate campaign to unravel domestic climate policy.
"The Climate Change Act was a ground-breaking attempt to take meaningful action on greenhouse gas emissions and the Lib Dems must not allow it to be compromised with a watered-down fourth carbon budget."
David Nussbaum, chief executive of wildlife charity WWF-UK, said: "Nick Clegg should be applauded first and foremost for making such a positive speech on the environment.
"This is especially welcome when some other senior politicians have been quiet - or even negative - about the opportunities for creating jobs and growth from protecting nature, tackling climate change and going green."