Theresa May will legally commit to ending the UK’s contribution to global warming by 2050 before she leaves No 10, but there are fears of a “get-out clause” that could allow her successor to roll back on the measures. Critics have also highlighted the lack of a detailed action plan.

The prime minister will announce legislation to set a path to “net zero” greenhouse gas emissions by the middle of the century, as recommended by her climate change advisers.

But there are concerns the move – an attempt to create a “legacy” achievement for a PM forced out of office for her Brexit failure – will fail to bind her successor to detailed action to help curb runaway climate change.

Friends of the Earth warned of “cynical gesture politics” if the legal commitment was not backed with real teeth in terms of policy and money.

Ms May is also under pressure from her chancellor to agree an “explicit review point”, allowing the next government to rethink the 2050 commitment if other countries fail to follow suit.

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In a leaked letter, Philip Hammond claimed the plan put forward by the Committee on Climate Change (CCC) – including an end to petrol and diesel cars and gas boilers and a huge shift to green energy, as well as drastic cuts in meat-eating – would cost more than £1 trillion.

He was immediately accused by green groups and opposition politicians of trying to block effective action by “putting ideology before our wellbeing”.

It appeared unlikely the legislation would have an action plan attached, as it can be achieved by simply changing the 2008 Climate Change Act through a new regulation.

The bid to make the UK a world leader on climate change comes after a poll for The Independent found overwhelming support, with 59 per cent of voters in favour of the net zero pledge and only 8 per cent against.

It could be introduced as early as next week – as Conservative MPs stage the first votes to find Ms May’s successor, with the winner due to move into Downing Street in late July.

A government source told The Independent the move was expected to have “broad parliamentary support”, with only a few Tory mavericks likely to voice opposition.

Emissions from some activities, including air travel and farming, are viewed as unavoidable by 2050, but “net zero” would be achieved by taking carbon out of the air by growing trees or burying carbon dioxide.

However, Mr Hammond’s letter undermined the plan by warning it would shrink the money available for schools, the NHS, the police and other public spending priorities.

The chancellor also argued the target would render some industries “economically uncompetitive” without huge government subsidies.

The CCC estimated that reaching net zero will cost £50bn a year, but the Department for Business, Energy and Industrial Strategy (BEIS) puts the figure at £70bn, according to the chancellor’s letter.

“On the basis of these estimates, the total cost of transitioning to a zero-carbon economy is likely to be well in excess of a trillion pounds,” he wrote.

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Mr Hammond called for a subsequent review to prevent “potentially damaging impacts” and the “explicit review point”, or get-out clause, if other countries fail to act.

Mike Childs, Friends of the Earth’s head of policy, said: “Setting a long-term target is crucial – but without short-term action it would represent nothing but cynical gesture politics. The government must then rapidly overhaul its woefully inadequate clean growth strategy and put the climate emergency at the heart of its forthcoming spending review.”

Greenpeace said legislating for net zero would be “hugely important, and will set both a clear direction of travel for government and the economy”.

However, Doug Parr, its chief scientist, criticised Mr Hammond’s letter, saying: “The Treasury is putting their ideology before our wellbeing and trying to shape the public debate for political ends.”

Ed Davey, a Liberal Democrat leadership candidate, warned: “Hammond might be trying to reclaim his crown as a fiscal hawk in the dying embers of May’s premiership, but this intervention is wrong headed and threatens our children’s future. The cost of tackling the climate emergency is massively outweighed by the long-term cost of not acting. The chancellor has got his sums wrong.”

The prime minister’s spokeswoman would not confirm she intended to legally commit the government to the 2050 target before leaving office.

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“In the week of the CCC report, we strongly welcomed the recommendations but set out that we would be formally responding as soon as possible. That remains our intention,” she said.

However, the spokeswoman appeared to take a swipe at Mr Hammond’s letter, saying: “There are a lot of figures out there on this issue that don’t factor in the benefits or consider the costs of not doing this.

“The costs relating to meeting this target are whole-of-economy costs, not a fiscal cost. So, it’s not really right to frame it as a trade-off for public spending.”