Climate aid to developing countries from the UK will be cut by about 14% to roughly £2bn a year under government plans, in a move critics said would put national security and lives overseas at risk.

The move follows bitter rows with the Treasury, which wanted deeper cuts owing to pressure on spending resulting from the war in Iran.

Overall, the UK’s aid budget was slashed to 0.3% of gross national income, with programmes on health, education and humanitarian assistance all facing the axe.

Climate spending will be “around” £6bn over three years, the government said before the announcement on Thursday. But experts told the Guardian this was likely to mean less than £6bn, rather than more. Under the previous five-year arrangement, the UK provided £11.6bn over five years, or about £2.3bn a year.

The previous earmark of £3bn in funding for nature and forest projects has also been scrapped.

The climate funding pledge abandons the previous practice of setting five-year budgets, to allow for longer-term projects of the kind that experts said were more efficient.

The Guardian understands the Treasury was arguing in key meetings last weekend that the overall aid budget should be slashed even further than the cuts, announced last year and only now being fully implemented, from 0.5% to 0.3% of gross national income.

The Treasury argued that more money was needed for defence and to shore up the economy because of the war in Iran.

Yvette Cooper, the foreign secretary, said: “At a time when conflict is raging in many parts of the world, we will maintain and protect our support to people in Ukraine, Sudan, Palestine and Lebanon, and match this with diplomatic action to prevent and resolve conflicts that are causing such devastation and humanitarian suffering.

“With less investment we need to refocus to ensure it has the most impact. Responding to desperate humanitarian crises, preventing conflict and upholding international law are not only a core part of Britain’s values and our common humanity. They are also central to Britain’s interests, because in an increasingly interconnected world, we know that instability abroad affects us back at home.”

Several Labour MPs voiced their concerns over the cuts. The chair of the all party parliamentary group on global health security, Dr Beccy Cooper, said: “Labour is, and always has been, a party of internationalism. When we step back from our shared commitments, we lose both our strength and our standing in the world. We are a soft power superpower and we should be proud of that.

“Today’s spending plans put Britain and the world at risk. When health systems in the poorest countries are not supported to become resilient, diseases spread faster and further. Protecting public health at home means investing in strong health systems everywhere.”

Gareth Thomas, a former international development minister, said: “In an already unsafe world, cutting aid risks alienating key allies and will make improving children’s health and education in Commonwealth countries more difficult. We risk creating more opportunities for regimes who don’t share our values. Our security depends not just on a stronger military but also on building soft power so that our soldiers aren’t needed.”

Zac Goldsmith, a former Foreign Office minister under the Conservatives, said: “This government’s own national security assessment was a stark warning that global biodiversity loss and ecosystem collapse are direct, systemic threats to UK security and prosperity, and yet nature has taken the brunt of the cuts. It’s unbelievably shortsighted. No wonder the government was so keen to suppress the report.”

Campaigners told the Guardian that cutting the budget for climate and nature aid was shortsighted, especially given a recent warning by intelligence leaders that the collapse of ecosystems around the world represented a serious threat to the UK’s national security.

Jonathan Hall, the managing director of Conservation International UK, said: “We simply cannot hope to stop the climate crisis without tropical forests. For 15 years, the UK’s international climate finance has always had a clear funding goal for stopping deforestation. To now drop any commitment to spend a significant portion of our climate aid on nature and forests flies in the face of the government’s own security experts, the latest scientific evidence and UK polling which shows that this is the most popular form of climate spending with the UK public.”