Boris Johnson said he is “increasingly frustrated” with world leaders at their commitments to tackle the climate crisis.
The prime minister warned during a meeting at the United Nations in New York that the gap between what industrialised nations have promised and what they are actually delivering remains “vast”.
He said rich countries were doing “nowhere near enough”.
Johnson urged them to redouble their efforts to hit a key financing pledge to help developing nations.
But he has conceded there is only a “six out of 10” chance of hitting before the UK hosts the Cop26 climate summit in November.
“Everyone nods and we all agree that something must be done,” Johnson told the UN meeting.
“Yet I confess I’m increasingly frustrated that the ‘something’ to which many of you have committed is nowhere near enough.
“It is the biggest economies in the world that are causing the problem, while the smallest suffer the worst consequences.
“And while progress is being made all over the world, the gulf between what has been promised, what is actually being delivered, and what needs to happen… it remains vast.
“Too many major economies – some represented here today, some absent – are lagging too far behind.”
Johnson issued a stark warning over what would happen if they do not redouble their efforts to hit the target of giving 100 billion dollars (£73 billion) a year in support to developing nations to cut their carbon emissions and protect themselves against environmental change.
“If you say that the lives of their children are not worth the hassle of reducing domestic coal consumption, will they vote with you in fora such as this?” he said.
“Will they work with you, borrow from you, stand with you if you tell the world that you don’t care whether their land and their people slip below the waves?
“To be merely a bystander is to be complicit in their fate – yet that is exactly what you will be if you fail to act this year.”
Johnson has downplayed the chances of hitting the 100 billion dollar target before he hosts Cop26 in Glasgow.
The prime minister will meet Amazon boss Jeff Bezos on Monday and has vowed to tell him the online giant must pay its fair share of taxes in the UK and address working standards for employees.