Boris Johnson is planning to convene last-ditch climate talks with the president of China, Xi Jinping, at a crunch meeting of world leaders later this month, in hopes of breaking the global impasse on climate action ahead of the Cop26 climate summit being hosted in Glasgow this November.

Xi will be invited, along with the leaders of about 30 other countries, to a high-level meeting on the sidelines of the UN general assembly in New York on 20 September, the Guardian has learned.

China is crucial to the success of Cop26, as the world’s biggest emitter of greenhouse gases, but is a focus of increasing concern as its current emissions plans are regarded as too weak.

Global experts on the talks told the Guardian the UK’s own actions – in slashing overseas aid and failing to persuade rich countries to provide more climate finance to help poorer countries – had stripped it of much of its power to influence China, imperilling the prospects of success at Cop26.

Tom Burke, a veteran adviser to governments and co-founder of the E3G green thinktank, said the “stupid” decision to cut overseas aid had removed a key diplomatic tool at the talks: “Boris Johnson has sent himself naked into the conference chamber. He has nothing much to offer.”

The informal meeting on 20 September, which will be a hybrid of virtual and in-person and co-hosted by the UN secretary general, Antonio Guterres, represents one of the last opportunities for top-level diplomacy with China, with just 50 days to go until Cop26. The meeting will focus on ways to reach the longstanding pledge by rich countries to provide $100bn a year, from public and private sources, to help poorer countries cut their emissions and cope with the impacts of climate breakdown.

China is responsible for more than a quarter of global carbon emissions and despite rapidly growing renewable power is still heavily dependent on coal and still planning new coal-fired power stations despite promises to reduce its use. The International Energy Agency (IEA) has said that if China expands its use of coal, annual global carbon emissions are likely to rise by a record amount next year, putting hopes of limiting global heating to 1.5C – the stronger of two goals in the 2015 Paris agreement – all but out of reach.

Jennifer Morgan, the executive director of Greenpeace International, said China was crucial to hopes of success at Cop26. “The world needs China to move to keep 1.5C alive. Action by China, as the world’s biggest emitter, is fundamental – decisions by the Chinese now have a huge influence on the wellbeing of the planet and our future.”

Xi surprised the world a year ago, in a speech to the UN general assembly, by pledging to peak China’s still-growing emissions before the end of this decade, and reach net zero carbon by 2060. These commitments marked a strong improvement on previous plans, but are still insufficient to ensure global emissions are halved by 2030, which scientists say is necessary to keep within 1.5C.

Morgan said China should agree to peak its emissions by 2025, set a specific timeline for phasing out its use of coal and introduce a moratorium on its financing of coal development overseas to give the world a chance of staying within 1.5C.

Underlining the importance of China, the US, UK and EU are all engaged in frantic diplomacy with the country’s leaders. Joe Biden’s special envoy on the climate, John Kerry, visited Tianjin this month and Alok Sharma, the UK minister who will preside over Cop26, followed last week.

The Guardian understands these meetings were positive, though they produced no new concrete measures from China. Xie Zhenhua, China’s top climate official, told Reuters the talks with Sharma were “candid, in-depth and constructive”.

However, all face an uphill task. Diplomatic relations between the US and China have deteriorated over trade spats, and with the UK over human rights and trade issues. Furthermore, the UK faces a specific problem in its ability to convene other developing countries to counterbalance China.

At previous climate Cops, before the landmark Paris agreement was signed in 2015, China was reluctant to commit to stronger climate action. This came to a head at a crunch meeting in Durban in 2011, where China and India were the last nations holding out on a deal that laid the groundwork for Paris.

In a dramatic standoff in the dying hours, the EU gathered together a “coalition of high ambition” made up of more than 130 developing countries, including the poorest and most vulnerable, to push for a resolution. This marked a significant change: developing countries that had previously looked to China for leadership decided their best interests lay in avoiding climate breakdown, rather than holding on to Beijing’s coattails.

China, stung by the defection of its former allies, finally agreed to sign the deal that led to the Paris agreement four years later. “China hated the coalition of high ambition,” said one former high-level diplomat.

Experts believe the UK should be trying to assemble a similar coalition now. But the decision to slash overseas aid has damaged Johnson’s standing in the eyes of poorer nations, and the failure so far to secure the long-promised $100bn a year in climate finance is also a key stumbling block.

Burke said: “The effective way to put pressure on China is to build a coalition of people that China is interested in, and the best tool to achieve that was the authority the UK had as one of the world’s biggest and most consistent providers of development aid. The prime minister has sacrificed that, and for nothing. He has thrown away one of the most potent weapons for delivering at Glasgow.”

Experts said that if China was to strengthen its climate plans, it would be for the country’s own reasons, rather than the result of UK, US or other external pleading. Morgan said: “This is not a favour that China offers to the UK – this is in China’s own self-interest.”

Bob Ward, policy director at the Grantham Research Institute on Climate Change at the London School of Economics, said finance was key. “China takes seriously its role in supporting other developing countries. It is likely waiting for an indication that the rich countries will honour their commitment to mobilise $100bn a year for developing countries from public and private sources.”

The UK is keenly aware of how central the $100bn pledge is to Cop26 and the Guardian understands this will be a key focus of the September meeting in New York. The presence of some of the countries most vulnerable to the climate crisis is also a message to China. With time running short, the UK is making a push across all fronts.

A UK official said: “We are pushing hard to finally hit the $100bn annual pot of money but there’s a number of rich nations that are holding out … We don’t have much time. That’s why the UK has convened this meeting at UNGA – Boris Johnson will give vulnerable nations plenty of space to challenge rich nations. Big emitters need to be left in no doubt what developing countries need to make Cop26 work for them.”

Additional reporting by Xiaoqian Zhu in Beijing