It would show the UK is not “serious about” helping to protect the world’s poorest countries from the devastating effects of global heating, a letter to Boris Johnson warns.
It has been sent by a panel known as the Friends of COP – appointed by summit president Alok Sharma – to advise the government and boasts experienced climate experts from around the world.
Mr Sunak is believed to be planning several “accounting tricks” that would further reduce the amount of UK money that is spent on humanitarian and development aid.
The same switch is likely to be made over the cost of giving Covid vaccines to developing countries and chunks of aid could be switched from day-to-day spending budgets to “capital” budgets.
The Centre for Global Development think tank, said the changes, if all made together, could cut the Foreign Office’s discretionary aid budget from £8bn to as little as £2bn.
Now the letter threatens huge embarrassment for the prime minister, who is already wrestling with fears that Cop26 will fail to reach a deal to prevent catastrophic climate change.
Seen by the BBC, it reads: “As ‘Friends of COP’ we are writing to you to express our deep concern at the prospect of further UK aid cuts in the final few days before COP26.
“The ability of the UK to act as a genuine, trusted partner for developing countries is of crucial importance to COP26’s success.
“Further implied cuts to overseas aid at the Comprehensive Spending Review (CSR) would send a signal that the UK is neither committed to, nor serious about, enabling a green global recovery from the pandemic, nor improving the resilience of the most vulnerable to climate change.”
Mr Sunak overcame a Tory backbench rebellion to ram through the current £4bn-a-year cuts, which are almost certain to continue until after the next general election.
The Friends of COP who have signed the letter include the heads of the Grantham Research Institute of Climate Change and the European Climate Foundation, plus a former Cop president and head of the UN framework convention on climate change.
But a Treasury spokesperson said the UK “is and will remain a world leader in international development”, spending more than £10bn a year.
The country will return to spending 0.7 per cent of national income – having slashed it to 0.5 per cent – “when the fiscal situation allows”.