Global fertiliser supplies must be freed up within weeks to avoid disaster, with harvests suffering and food prices rising, the UK’s foreign secretary, Yvette Cooper, has said.

The war in Iran has frozen shipments of fertiliser through the strait of Hormuz, creating a supply crunch that has already damaged farming in the UK, Europe and the US and is having its worst impacts in the developing world, where farmers cannot afford the higher prices now being charged.

“The world is sleepwalking into a global food crisis,” Cooper said. “We cannot risk tens of millions of people going hungry because one country has hijacked an international shipping lane.”

Spring is the crucial planting time – if farmers in the northern hemisphere cannot be assured of fertiliser supplies now, the damage will play out over the next year.

“Iran’s continued closure of the strait of Hormuz while the agricultural clock is ticking shows why we need urgent global pressure to get the strait reopened, fertiliser and fuel moving, and ease the costs of living pressures,” Cooper said before a conference of governments in London to discuss overseas aid and development.

“This crisis is affecting developed and developing countries, the private and public sectors alike,” she said. “It shows why we need a new approach to global partnerships, to drive international development to prevent crises in the first place. The world has changed faster than the international system can support it.”

The Global Partnerships conference, jointly hosted by the UK and South African governments and supported by the Children’s Investment Fund Foundation, is intended to help governments, private sector investors and civil society find new ways of working together.

Despite increasing pressure on prices and rising mountains of debt in developing countries, many rich countries are reducing their overseas aid. The UK cut aid from 0.5% of gross national income under the last government – itself a cut from 0.7% under the previous Labour government – to 0.3%, while the US under Donald Trump has dismantled the USAID agency.

The World Food Programme estimates that almost 45 million more people could fall into acute food insecurity if the Iran conflict does not end by the middle of this year.

Cooper said overseas aid was in the UK’s national interest. “Instability abroad affects us here at home, from energy prices to food security. Building resilience abroad makes the UK stronger,” she said.

The UK’s spy bosses warned in a report that the government has so far published only partially that the collapse of key ecosystems in developing countries, partly driven by the climate crisis, would have devastating impacts on the UK’s national security. However, the government has not publicly discussed the report or its response to it.

Climate finance, which helps developing countries protect their ecosystems and become more resilient to the impacts of extreme weather, has been cut to £2bn a year over the next three years.

Jenny Chapman, the minister for development at the Foreign, Development and Commonwealth Office, told the Guardian that partnerships with the private sector could more than double the aid available.

“We are absolutely not backing away from our contributions and our responsibilities,” she said. “We can get more climate finance by moving the way work on it. We need to make a lot more impact than we have done.”

At the conference, British International Investment – an agency that uses public money to invest with the private sector, and a conference co-host – will announce £4.6bn for climate investment in emerging markets. There will be $250m for the African Development Bank and an increase in the UK’s shareholding in the Inter-American Development Bank.

Cooper will announce a new health partnership to support children injured in Gaza, and more support for health systems to develop new medicines and vaccines more quickly, as well as a £200m investment in science and technology. The UK is to be the next president of the G20 group of developed and developing economies.