Some of the UK's most influential charities have today written to the Chancellor Sajid Javid warning he must double government investment in climate and nature to avert a climate crisis and keep the UK on track to meet its carbon reduction goals.

The letter, sent to the Chancellor as he prepares for Wednesday's Spending Review, calls for a 'climate emergency budget' that brings spending on climate and nature up from £17bn a year currently to at least £42bn.

The charities warn failing to invest enough in the low-carbon transition now will leave future generations with a "planet-sized debt", while ramping up spending would "unleash" a clean industrial revolution that will bring green growth to the UK.

The letter welcomes the government's 2050 net zero emissions goal legislated by Theresa May's government, but says the net zero date must be brought forward and accompanied by a "step up in policy and financial commitments" from government. The government's own analysis suggests it is not on course to meet its net zero commitments.

"The Spending Review and Budget provide the first moments for climate action under your leadership," the letter reads. "The climate and environmental crisis has no modern parallel and it demands your - and all of your government's - urgent attention. It needs to be treated as a long term investment in the future like education; the UK spends over £41bn on secondary education alone. Your department holds the key to ending the UK's contribution to climate change and reversing nature's retreat, while also setting off a positive economic transformation."

Signatories to the letter include Amnesty International, Christian Aid, Friends of the Earth, Green Alliance, Greenpeace, Oxfam, The RSPB, and WWF UK.

Their analysis of the funding needed is backed by a new report also released today by Greenpeace, which suggests over the next three years green investment must be brought up to around five per cent of overall government spending, roughly the same amount spent currently on secondary education and defence.

That would pay for energy efficiency improvements for homes, a more comprehensive and lower carbon public transport system, and nature restoration to reduce flooding risk, they say. Much of the cash could be raised by cutting fossil fuel subsidies, steering funds away from high-carbon projects such as road building towards lower carbon schemes, and taxing polluting activities such as flying.

An accompanying poll carried out by Opinium on behalf of Greenpeace suggests more than half of people think the government should be spending more to address climate change.

Craig Bennett, chief executive of Friends of the Earth, said politicians must "put their money where their mouth is" when it comes to the climate crisis. "Introducing a climate emergency budget will mean we can invest in creating warm homes, clean air, easy ways for people to get around and a thriving UK renewable industry," he said. "This is the investment Britain is crying out for."

The new Prime Minister Boris Johnson has repeatedly backed the net zero target and promised Britain will become a leader in green technologies. In an interview with The Sunday Times published yesterday he said he wants his administration to be "the greenest ever". "We're going to do extraordinary things on the environment," he promised.

But the first few weeks of his premiership have been dominated by political wrangles over Brexit and a series of funding pledges on education, crime and social care.  

The Treasury was considering a response to the letter at the time of going to press. 

Wednesday's Spending Review has been downgraded from a full government Spending Review to just catering for a single year - leading many commentators to regard it as an emergency Brexit Budget. However, its short time span has not stopped calls from all sectors of the economy for more cash.

Leaders of England's biggest cities also today called on the government to prioritise air pollution in the Spending Review, calling on the Chancellor to deliver an "enhanced Clean Air Fund" to bring air quality back to legal levels across the country.

The Royal College of Physicians suggests health problems associated with air pollution cost the NHS more than £20bn per year, yet currently only six local authorities have plans to introduce Clean Air Zones to tackle the problem.

Mayors and political leaders from London, Manchester, Birmingham, Bristol, Cambridge, Leeds, Leicester, Liverpool, Newcastle, Nottingham, Oxford, Sheffield, Southampton and the West of England Combined Authority say a national network of 30 Clean Air Zones could be unlocked with £1bn of extra funding from Treasury, matched by £500m from business.  

"Air pollution is a national health crisis which is responsible for tens of thousands of premature deaths around the country and costs over £20bn a year," Mayor of London Sadiq Khan said. "Cities including London cannot deliver further Clean Air Zones without urgent government funding."

"Everyone deserves the right to breathe clean air and the Chancellor simply cannot afford to delay immediate action on this invisible killer."