More than 100 world leaders - including Brazil, Russia and Indonesia - have backed a landmark deal to protect and restore the Earth's rainforests.
On the second day of the COP26 climate change summit in Glasgow on Tuesday, leaders covering 85% of the world's forests will commit to halt and reverse deforestation and land degradation by 2030.
Downing Street said the pledges were backed by £8.75bn of public funding with a further £5.3 billion in private investment.
More than 30 financial institutions handling trillions of pounds of assets also pledged to stop investing in activities linked to destroying forests.
The commitment, to be formally announced at an event convened by Boris Johnson, has been welcomed by campaigners and experts, in particular the recognition of the role of indigenous people in protecting forests.
But there were warnings that commitments needed to be delivered on, and standing forests must be protected, as well as there being a focus on restoring forests.
The Prime Minister backed the move, saying it would support the Cop26 goal of restricting global warming to 1.5C through the absorption of carbon emissions by forests.
"These great teeming ecosystems - these cathedrals of nature - are the lungs of our planet," he was expected to tell the event. "Forests support communities, livelihoods and food supply, and absorb the carbon we pump into the atmosphere. They are essential to our very survival.
"With today's unprecedented pledges, we will have a chance to end humanity's long history as nature's conqueror, and instead become its custodian."
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It came as Boris Johnson warned world leaders they will be judged with "bitterness and resentment" by future generations if they fail to tackle global warming.
The Prime Minister opened the Cop26 summit in Glasgow with an urgent plea to fellow leaders to help "defuse the bomb" of climate change.
Watched by US President Joe Biden and more than 100 other leaders, he painted an apocalyptic picture of the future with wildfires, droughts and flooded cities.
The Government has tried to lower expectations that the climate conference will achieve its aim of slowing down global warming to 1.5C.
In his opening remarks, Mr Johnson claimed that children would be the harshest judges of their failure to act now.
"We are now coming centre stage before a vast and uncountable audience of posterity and we must not fluff our lines or miss our cue," he said.
"Because if we fail, they will not forgive us - 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 added: "The longer we fail to act the worse it gets and the higher the price when we are eventually forced by catastrophe to act."
As climate activist Greta Thunberg addressed campaigners outside, he admitted her claims that previous promises were "nothing but blah blah blah" were right.
But Mr Johnson faced charges of hypocrisy after it emerged he would be flying back to London after the summit tonight.
Downing Street claimed the PM faced "significant time constraints" when asked by the Mirror if saving time was more important than saving the environment.
His spokesman said the plane was one of the most environmentally-friendly of its kind because 35% of its fuel is described as sustainable.
Sir David Attenborough warned leaders that humanity is "already in trouble" due to burning fossil fuels, destroying nature and releasing carbon into the atmosphere.
"Is this how our story is due to end - a tale of the smartest species doomed by that all too human characteristic of failing to see the bigger picture in pursuit of short-term goals?" he asked.
"Perhaps the fact that the people affected by climate change are no longer some imagined future generations but young people alive today.
"Perhaps that will give us the impetus we need to rewrite our story, to turn this tragedy into a triumph."
UN Secretary General António Guterres warned humanity was "digging our own graves" through continued burning and drilling and mining.
He warned the planet was "careering towards climate catastrophe" with recent pledges failing to go far enough.
Mr Johnson admitted that world leaders had already "flunked" pledges to meet $100bn to help vulnerable nations tackle climate change.
India dealt another blow to his hopes by announcing it would only cut carbon emissions to net zero by 2070 - 20 years after the 2050 target.
British officials admitted they were "surprised" by the delay - suggesting the UK could even try to meet net zero five years early to persuade other nations.
China and Russia, whose leaders have stayed away from the summit, along with Saudi Arabia will miss the target by 10 years.
But Joe Biden pledged that America was "back at the table" in the fight against global warming after his predecessor Donald Trump withdrew from global deals.
The US president, who was filmed dozing off during the speeches, said: "Let this be the moment when we answer history's call, here in Glasgow.
"Let this be the start of a decade of transformative action that preserves our planet and raises the quality of life for people everywhere.
"We can do this, we just have to make a choice to do it. So, let's get to work."
