Rishi Sunak was warned that he faced exclusion from key discussions on the climate among world leaders at the UN before he decided to snub a global summit later this month. It was announced last month that Sunak would be the first prime minister in a decade to avoid attending the annual UN general assembly gathering of world leaders. The reason given was his busy schedule, but the Guardian has learned that turning up risked severe embarrassment for the prime minister. The UN secretary-general, António Guterres, has called on world leaders to attend his climate ambition summit, on the sidelines of this year’s UN general assembly meeting. Leaders of the world’s biggest economies, including Joe Biden of the US, are expected to go. But Guterres has stipulated that only countries that can show they have ambitious policies to reduce their emissions in line with the goals of the Paris agreement would be allowed to participate in the climate ambition summit, the segment of the meeting of world leaders dedicated to the climate crisis. World leaders received letters from the UN last month making these terms clear, the Guardian understands. They were told that merely having net zero targets would not be sufficient to guarantee participation, and evidence of clear and ambitious policy measures and commitments was needed. Without this, countries could attend but not participate in the discussions. The climate ambition summit will take place at the UN headquarters in New York on 20 September, after heads of state and government meet for more general talks on 18 and 19 September. The UK government has caused consternation in capitals around the world by seeming to waver on its commitments to net zero emissions and the 2015 Paris climate agreement. Sunak’s decision over the summer to “max out” the North Sea by licensing new oil and gas fields raised eyebrows among many countries and climate diplomats. Sunak would risk falling foul of the UN secretary-general’s strict stipulations if he were to attempt to participate in the climate segment. The potential embarrassment he faced throws a new light on his decision not to attend the UN general assembly. The foreign secretary, James Cleverly, and deputy prime minister, Oliver Dowden, are expected to attend the UN general assembly in place of Sunak. Leaders of most of the world’s 20 biggest economies are also meeting in Delhi this weekend, as India holds the presidency of the G20 group. The climate crisis is expected to be discussed after a summer of record temperatures in many parts of the world, but no diplomatic breakthrough is expected there. One close observer of climate talks said: “The climate summit on 20 September is invite-only – [countries have to demonstrate] progress to play. The bar is high. If Sunak wants to be in the climate photo he needs to lead and value the fact that this is a race and it’s a race we all must win.” Ed Miliband, the shadow net zero secretary, warned of the damage to the UK’s standing. “If it is the case that Rishi Sunak has been excluded from global climate discussions, it will confirm what we in the UK already know – that he is a prime minister who is trashing the UK’s reputation of climate leadership, just like he has trashed our nation’s clean power resources,” he said. A No 10 spokesperson said: “The UK delegation at the UN general assembly high-level week will be led by the deputy prime minister, accompanied by the foreign secretary and others. “The prime minister is expected to hold discussions with a number of world leaders in the coming weeks, including at the G20 summit in New Delhi and the Cop28 summit in the UAE. He and other ministers will continue to use all their engagements with their international counterparts to drive forward the government’s priorities, including on growing the economy, stopping illegal migration, fighting climate change and supporting Ukraine.”