The veteran broadcaster and environmentalist will address the leaders gathered in Cornwall on Sunday as they set out plans to reverse biodiversity loss and to fund infrastructure development around the world.

Boris Johnson is also launching a Stg 500 million ($A916 million) "blue planet fund" to protect the world's oceans and marine life.

The leaders of the G7 - UK, US, Canada, Japan, France, Germany and Italy - will make a series of environmental commitments in Carbis Bay

Attenborough will deliver a message to the G7, plus guests Australia, India, South Korea and South Africa, at a session on climate and nature.

In advance of the session, he said: "The natural world today is greatly diminished. That is undeniable.

"Our climate is warming fast. That is beyond doubt. Our societies and nations are unequal and that is sadly plain to see.

"But the question science forces us to address specifically in 2021 is whether as a result of these intertwined facts we are on the verge of destabilising the entire planet?

"If that is so, then the decisions we make this decade - in particular the decisions made by the most economically advanced nations - are the most important in human history."

G7 nations are expected to commit to almost halve their emissions by 2030 relative to 2010. The UK has already pledged to cut emissions by at least 68 per cent by 2030 on 1990 levels, the equivalent to a 58 per cent reduction on 2010 levels.

The countries will set out the action they will take to slash carbon emissions, including measures such as ending all unabated coal use as soon as possible, halting almost all direct government support for the fossil fuel energy sector overseas and phasing out petrol and diesel cars.

The G7 will also endorse a nature compact, aimed at halting and reversing biodiversity loss by 2030 - including supporting the global target to conserve or protect at least 30 per cent of land and oceans by the end of the decade.

Australian Associated Press