The amount of wildlife in the world's forests has plummeted by more than half (53 per cent) in just over 40 years, conservationists have found.
Humanity is killing the Earth's greatest natural ally in the fight against climate breakdown, our forests, according to the report by the WWF.
The charity is calling on world leaders to declare a planetary emergency and develop a "new deal for nature and people" to halt climate breakdown, restore nature and fix food systems.
The first ever global assessment of forest biodiversity shows that habitat loss and degradation, chiefly caused by people, account for 60 per cent of the threats to forests and forest species.
The report, 'Below the Canopy', written jointly by WWF and ZSL, found the drops in wild animal and bird populations were greatest in tropical forests such as the Amazon rainforest, where there is the most wildlife to lose.
Monitored populations of forest-living birds, mammals, amphibians and reptiles declined, on average, by 53 per cent between 1970 and 2014, the most recent year with available data.
Protecting and restoring forests must be at the heart of the global plan, the charity says.
WWF says that forests, which are home to more than half of the world's land-based species, are vital to the health of the planet, absorbing damaging greenhouse gases.
The report outlines how in the vast tropical forests of South America and Africa, the carbon locked in would decline if large birds and primates in particular were lost.
"When animals are lost from forests this has severe implications for forest health, the livelihoods of more than a billion humans who depend on forests, and our opportunity to mitigate against climate disaster," WWF says.
Deforestation and forest degradation account for about 10 per cent of global greenhouse gas emissions.
Some scientists argue that an irreversible transformation of the Amazon is close as already about a fifth has been cleared, close to the tipping point of 20-25 per cent, where some rainforest could dry out into a savannah.
The report authors want heads of state to develop a new global agreement at the 75th UN General Assembly meeting next year, when they are expected to negotiate new 10-year targets for the Convention on Biological Diversity.
Will Baldwin-Cantello, a WWF forests specialist, said: "Forests are complex systems that depend on the wildlife that live in them to keep them healthy, and the rapid decrease in forest wildlife in recent decades is an urgent warning sign.
"Forests are our greatest natural ally in the fight against climate breakdown. We lose them at our peril.
"We need global leaders to declare a planetary emergency and kickstart a global programme of recovery to keep our forests standing to protect our planet."