There is "no chance" of an agreement to prevent runaway climate change at Cop26 given current carbon-cutting pledges, Boris Johnson has admitted.
One day before the crucial summit in Glasgow, an increasingly gloomy prime minister appealed to countries to "get their act together" to rescue the event from the risk of total failure.
"Where we stand today, there is no chance of us stopping climate change next week," Mr Johnson said, speaking at the G20 summit in Rome.
"There is no chance of us getting an agreement next week to limit climate change to 1.5 degrees.
"What we could conceivably do if everybody gets their act together, what we could do, is get an agreement that means that Cop26 in Glasgow is a way station that allows us to end climate change."
The comments come after the prime minister failed to persuade Chinese premier Xi Jinping to beef up the commitment to curb CO2 emissions by the world's biggest offender.
Beijing has disappointed No 10 by refusing to budge on its plan to reach peak emissions no later than 2030 - rejecting Mr Johnson's plea to bring that forward to 2025.
With Mr Jinping skipping the summit, there appears little prospect of a deal to prevent global temperature rises of more than 1.5C since industrialisation.
Currently, the planet is "way off track", the United Nations has warned, on a path to 2.7C - and, experts say, 2.1C even if existing CO2-cutting commitments are kept.
Just 7.5 per cent would be chopped off predicted annual greenhouse gas emissions in 2030 - far from the 47 per cent reduction that is needed.
India, Saudi Arabia and Turkey are among other major emitters that have yet to submit new plans for cutting emissions by 2030 to the UN.
Mr Johnson also repeated his apocalyptic warning that civilisation could collapse "like the Roman Empire" unless the climate emergency is averted.
Speaking in the famous Colosseum, he said: "The Roman Empire, they weren't expecting it and they went into reverse - we had a Dark Ages. It's important to remember things can get dramatically worse."
The prime minister's spokesman denied he was increasingly gloomy, insisting he had always believed that success at Cop26 was "an extremely difficult challenge".
However, on Friday, No 10 said its aim for the summit was commitments to "halve emissions by the end of this decade", to keep a 1.5C temperature rise within sight.
The UK is declining to push for a tougher "rachet" mechanism - to force countries to prove progress towards announced goals every two years, instead of five - apparently fearing that would not be agreed
And Cop26 will not attempt to agree a specific figure for reducing expected gigatonnes of carbon dioxide emitted by 2030, which will await further summits.
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