Rishi Sunak’s promise to meet an £11.6 billion climate and nature pledge looks set to be missed, according to a leaked internal memo.

The president who oversaw the COP26 climate conference in Glasgow has warned against the move.

The briefing note to ministers sets out that the international funding commitment would be a “huge challenge” and require backing for other aid projects to be slashed.

The UK Government insisted it is delivering on the pledge and said suggestions the commitment could be dropped are “false”.

Last week, the UK Government's statutory advisers, the Climate Change Committee, warned Tory ministers have undermined the careful language” used to negotiate a global climate pact at COP26 in Glasgow over its insistence to expand oil and gas development.

It added that the UK has “sent confusing signals on its climate priorities to the global community”.

The memo, obtained by the Guardian, says the commitment to provide £11.6 billion between April 2021 and March 2026 was made at a time when the Government was meeting its legally-enshrined target of spending 0.7% of national income on overseas aid.

That commitment was dropped to 0.5% as a result of the impact of Covid-19 on the nation’s finances.

The leaked document said meeting the climate pledge within this spending “would squeeze out room for other commitments such as humanitarian and women and girls”.

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The promise was made by Boris Johnson in 2019 but the BBC reported the memo pointed to “subsequent turbulence” in the economy – such as the pandemic – which had “turned a stretching target into a huge challenge”.

Meeting it would require a “reorientation” of the budget on a scale which “has not previously been achieved”.

Ministers including Rishi Sunak have publicly declared the £11.6 billion commitment remains in place.

A UK Government spokesman said: “Claims that the international climate finance pledge is being dropped are false.

“As the Prime Minister set out at Cop27, the Government remains committed to spending £11.6 billion on international climate finance and we are delivering on that pledge.

Read more: Climate advisers: UK has 'undermined' COP26 progress with oil push

“We spent over £1.4 billion on international climate finance over the course of the 2021/22 financial year, supporting developing countries to reduce poverty and respond to the causes and impacts of climate change.

“We will publish the latest annual figures in due course.”

Lord Goldsmith, who raised the issue when he resigned as a minister last week, said failing to meet the promise would be seen as a “betrayal” around the world.

“The PM is insisting he isn’t breaking his promise,” the peer said. “The figures show he is.”

He said the only way the commitment could be met is if the next government, in its first years in office, allocates “over 80% of all UK bilateral aid” to climate funding, at the expense of humanitarian, health and education schemes “which obviously it cannot do”.

Read more: Tory peer Zac Goldsmith quits UK Government over PM's climate 'apathy'

“There will be some who welcome this,” Lord Goldsmith said.

“But they should consider the impact on the UK of breaking a promise that Commonwealth allies and countless others prize above all others.

“It will be seen as an act of betrayal on a profound level and will cause us irreparable reputational harm.”

Senior Tory Sir Alok Sharma, president of the COP26 climate summit, warned against dropping the pledge.

“So hope the government is not planning to drop its climate finance pledge to some of the most climate vulnerable countries in the world,” he said on Twitter.

When Mr Johnson announced the commitment there was “spontaneous applause” and “it was a proud moment for the UK”, Sir Alok said.

Izzie McIntosh, climate campaigner at Global Justice Now, said: “Rishi Sunak was already set to plumb new depths of climate incompetence by giving the green light to new coal, oil and gas in this country.

"Now it seems he’s planning to scuttle the UK’s contribution to the global climate finance that is urgently needed and already long overdue. This would be a shameful failure.

“The UK is one of the world’s biggest historical emitters, and home to two of the largest oil companies in BP and Shell. The Government could meet this pledge many times over if it was strong enough to make these polluters pay.

"But Rishi Sunak seems to have stopped caring what happens after the next election. In climate terms, he is in a burning building pouring fuel on the fire on his way out the door.”