The G20 is failing poor and vulnerable countries by not agreeing to a climate plan that would ensure their people’s survival, leading figures at the Cop26 climate talks have said.

Leaders representing more than a billion of the people most at risk from the climate crisis told the Guardian they were “extremely concerned” and had hoped for more from the G20 summit in Rome.

They said the prospect of limiting global heating to 1.5C above pre-industrial levels, a vital threshold that scientists say is a “planetary boundary”, was slipping away as the UN conference opened in Glasgow.

Gaston Browne, the prime minister of Antigua and Barbuda and chair of the Alliance of Small Island States, which represents 39 countries, said: “From what I’ve seen it appears we are going to overshoot 1.5C. We are very concerned about that. This is a matter of survival for us.”

He blamed the influence of powerful private-sector interests for the G20’s failure to come up with better plans, and said developed countries would also suffer the consequences of climate breakdown. “We are here to save the planet, not to protect profits. There are very powerful multinational firms and lobbies … who benefit from fossil fuel subsidies.”