Mr Johnson will say during the virtual event: “We must turn these words into action and use them to build momentum, to agree ambitious goals and binding targets.
“We must act now, right now. We cannot afford to dither and delay because biodiversity loss is happening today and it is happening at a frightening rate.
“Left unchecked, the consequences will be catastrophic for us all. Extinction is forever - so our action must be immediate.”
Environmental groups welcomed the commitment, but called on the UK government to invest in existing protected sites and put the new pledge into law.
Dr Richard Benwell, chief executive of The Wildlife and Countryside Link, said: “Just eight per cent of England is currently protected for wildlife, so designating 30 per cent of land to restore nature would be a tremendous step forward…
“Of course, designation alone isn’t a guarantee of change. As with marine protection and existing terrestrial protected sites, strong management and investment are also needed.”
WWF-UK chief executive Tanya Steele said the announcement must be “backed up by urgent ambition”, calling for “strong legislation to avoid damaging trade deals and to stop the food we eat from destroying the environment”.
“Only then can we meet our climate targets, put nature on the path to recovery and set our sights on global leadership at Cop26,” she added.
Earlier this month, the UN announced that the world had failed to meet a single target to stop the destruction of nature. The WWF and the Zoological Society of London also released damning reports and studies in recent weeks, revealing the global populations of mammals, birds, fish, amphibians and reptiles plunged by 68 per cent on average between 1970 and 2016.