UK Prime Minister Boris Johnson opened the COP26 climate summit by warning leaders that humanity was "running down the clock" on the climate emergency, urging immediate action before it is too late.

Welcoming delegates to the high-profile Glasgow summit on Monday, Johnson invoked James Bond, comparing fighting global warming to the spy defusing a ticking time bomb.

"The Doomsday device is real and the clock is ticking to the furious rhythm of hundreds of billions of turbines and systems … covering the Earth in a suffocating blanket of CO2," he said.

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"Humanity has long since run down the clock on climate change. It's one minute to midnight on that Doomsday clock and we need to act now," said Johnson. Adding: "The longer we fail to act, the worse it gets and the higher the price when we are forced to act."

Delayed by a year because of the COVID-19 pandemic, COP26 aims to keep alive a target of capping global warming at 1.5 degrees Celsius above pre-industrial levels – a level scientists say would avoid its most destructive consequences.

However, current pledges to cut emissions would allow the planet's average surface temperature to rise by 2.7 degrees Celsius this century, which the United Nations says would rapidly exacerbate the destruction already being wrought by climate change.

Johnson went on to stress the importance of such action if leaders wanted to overturn a damning verdict from future generations.

"The anger and the impatience of the world will be uncontainable, unless we make this COP26 in Glasgow the moment when we get real about climate change, and we can get real on coal, cars, cash and trees," he said in the keynote speech.

Johnson echoed climate campaigner Greta Thunberg, who is also in Glasgow with thousands of other protesters, in calling for the summit not to indulge in "blah, blah, blah."

If the leaders "fluff our lines or miss our cue," future generations "will not forgive us," the prime minister said.

"They will know that Glasgow was the historic turning point when history failed to turn.

"They will judge us with bitterness and with a resentment that eclipses any of the climate activists of today - and they will be right," he concluded. 

As Johnson was speaking, Thunberg retweeted an appeal for her supporters to sign an open letter accusing global leaders of betrayal.

"This is not a drill. It's code red for the Earth," it read. "Millions will suffer as our planet is devastated  – a terrifying future that will be created, or avoided, by the decisions you make. You have the power to decide."