Ministers should tighten the UK's official climate change target - or face the courts, the government's former chief scientist has said.
Prof Sir David King is supporting a legal case forcing ministers to shrink carbon emissions to zero by 2050.
He says the current government goal - an 80% emissions cut by the same date - is too weak to protect the climate.
Ministers have promised more ambitious climate policies in their forthcoming and long-delayed Clean Growth plan.
But Prof King told BBC News the government knew the 80% target cut behind that plan was too weak.
Some 18 months ago, Environment Secretary Andrea Leadsom promised a 100% emissions reduction to keep the UK on track with its obligations under the 2015 Paris climate deal.
But ministers have failed to enshrine that 100% cut within the Climate Change Act.
"This is crazy," Prof King told the BBC. "The government knows very well what needs to be done - but it isn't doing it.
"If it takes legal action to force ministers to behave properly, then so be it - I'll support it."
Prof King is backing a preliminary legal action by a tiny group, Plan B, run by former government lawyer Tim Crosland.
It argues that Business Secretary Greg Clark is obliged under the act to tighten targets if the science shows it is needed. This is the basis of the case.
Mr Crosland has written to Mr Clark and says if there is no satisfactory reply after 14 days, he will take the case to the High Court for judicial review.
"The science has clearly hardened since the Climate Change Act was agreed," he said.
"If scientists are telling us our current course of emissions potentially takes us to catastrophe, then to stick to the current course is irrational.
"The best available science tells us the risks of crossing tipping points rise very sharply between 1.5 and 2C. And that means the UK cutting emissions to zero."
His case would be argued in court by Jonathan Crow, Attorney General to Prince Charles, and a former senior Treasury lawyer.
Mr Crosland says his co-claimants are a rabbi "who learned not to ignore a humanitarian crisis"; young people fearing for the future; and a supporter from Mauritius representing island states at risk from rising sea water.
Other groups are similarly frustrated with long delays in the government's climate strategy, and some are also considering legal action.
There are fears, though, that pushing for a zero emissions strategy when the government cannot yet reach its lower target may be counter-productive.
ClientEarth, one of the UK's most successful environment groups over the past decade, has pioneered the use of the courts to deliver environmental policies.
Jonathan Church, a climate lawyer with the group, said: "We hope that Plan B's claim will help draw attention to the urgency of the challenge we face and ensure that our government keeps its eye on the goals agreed in Paris.
"But targets do not on their own reduce emissions."
He added that the focus now should be ensuring that the Climate Change Act fulfils its purpose.
The government said it would consider Plan B's letter and respond in due course, adding: "The UK is a global leader on climate change".
A spokesperson said the forthcoming Clean Growth plan, which will outline how reduce emissions will be reduced, would be "ambitious and robust... and build on the economic opportunities across the country".
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