Ministers have been accused of abandoning nature in the pursuit of net zero as a row escalates over Britain’s refusal to back a flagship fund to save the rainforests.

Brazil had been assured Britain would support its Tropical Forest Forever Facility, a $125 billion investment scheme launched last week at the Cop30 climate conference in Belém.

British officials had even provided technical support to help design the fund, in which investors will purchase bonds that in turn fund the conservation of critical rainforests in poor countries. But No 10 blocked UK investment before the budget later this month.

Lord Goldsmith of Richmond Park, the former Conservative environment minister, said the decision was short-sighted. “Any climate plan that doesn’t include nature is just a total waste of time,” he said. “You could cover every single rooftop in Europe with solar panels, but if we lose the Congo or the Amazon or southeast Asia’s forests, we’re stuffed.”

The plight of the rainforests will be highlighted by a London School of Economics report to be released in Belém tomorrow warning that deforestation and climate change in the Congo’s rainforests could soon mean that more carbon dioxide is released there than absorbed.

Bob Ward, policy director of the Grantham Research Institute at LSE and co-author of the report, said that poses “serious dangers to lives and livelihoods across the world, including Britain”.

He added: “The failure to provide financial support for the Tropical Forests Forever Facility is without serious justification and clearly not in the best interests of the UK or the rest of the world.”

Lord Stern of Brentford, a renowned economist and climate expert, called the decision, apparently made at the last minute, a “serious mistake”.

Government insiders said the decision not to invest in the fund, after months of work behind the scenes to help set it up, was a diplomatic disaster.

“We were leading the charge on this and then we stepped away at the last minute,” one official said.

Ed Miliband, the energy secretary, who was in Belém with Starmer and will return this week for Cop30’s conclusion, admitted the proximity to the budget had scuppered plans for investment. “We’ve got a big fiscal event coming up in the next three three weeks and that’s obviously a very, very difficult context,” he told Christiana Figueres, the former UN climate chief, on the podcast, Outrage + Optimism. “That’s why we weren’t able to announce an investment now, but we certainly haven’t ruled it out for the future.”

We were leading the charge on this and then we stepped away at the last minute

The Department for Energy Security and Net Zero said: “The UK is incredibly supportive of the Tropical Forests Forever Facility and is proud to have played a part in assisting Brazil to develop the initiative. That support will continue, including by efforts to unlock private investment for the facility.”