Boris Johnson has described the latest warnings from UN scientists about the extent of the climate crisis as “sobering reading” that should provide the world with a wake-up call ahead of the Cop26 summit.

With the global climate conference due to open in Glasgow in less than three months, the British prime minister said he hoped the report from the Intergovernmental Panel on Climate Change (IPCC) would highlight the need for action now.

Ministers fear the UK-hosted conference, which has been billed as a landmark moment in the global effort to reduce emissions, may end up being seen as a damp squib. It is meant to be the moment that countries set out further details of how they will contribute to the goal of limiting global temperature rises to 1.5C above pre-industrial levels, but some of the world’s largest carbon emitters, such as China and India, have yet to submit detailed plans.

Commenting on the IPCC report, Johnson said: “Today’s report makes for sobering reading, and it is clear that the next decade is going to be pivotal to securing the future of our planet.

“We know what must be done to limit global warming – consign coal to history and shift to clean energy sources, protect nature and provide climate finance for countries on the frontline.”

Johnson also said he hoped the report would be “a wake-up call for the world to take action now, before we meet in Glasgow in November for the critical Cop26 summit”.

Alok Sharma, the cabinet minister who is president of Cop26, has been travelling the world encouraging countries to update their nationally determined contributions (NDCs) – their policy pledges to cut emissions. More than 110 countries have submitted updated NDCs with commitments stretching to 2030, but some of the biggest economies have not done so, and other plans are considered inadequate by climate experts.

Sharma said: “Our message to every country, government, business and part of society is simple. The next decade is decisive: follow the science and embrace your responsibility to keep the goal of 1.5C alive.”

The Labour party leader, Keir Starmer, said the IPCC report was “the starkest reminder yet that the climate crisis is here right now and is the biggest long-term threat we face”. He also accused Johnson of “failing to treat the crisis with the seriousness it deserves”.

The Lib Dem leader, Ed Davey, also accused Johnson of complacency, claiming the UK was “showing other countries that climate inaction is acceptable”.

Connor Schwartz, the climate lead at Friends of the Earth, said: “If the government wants to show they respect the world’s leading scientists on climate chaos, they can start by cancelling the Cambo oilfield, scrapping the coalmine in Cumbria, and ending UK funding for the mega-gas project in Mozambique; they can do that today.”

Extinction Rebellion (XR) marked the publication of the IPCC report by announcing that from Monday 23 August it will stage protest events in London for two weeks, aimed at disrupting the City and what it describes as “the root cause of the climate and ecological crisis – the political economy”.

Clare Farrell, an XR co-founder, said: “We are in the midst of a collective act of global, social evil, which is unprecedented in all of history. We spend more time measuring it than trying to stop it: this is in and of itself a crime.”

Referring to a recent briefing by Allegra Stratton, the prime minister’s spokesperson for Cop26, Farrell said: “This government is a joke, telling us how to wash our dishes when they should be leading the world towards a mobilisation that saves humanity.”