Boris Johnson is promising to protect 400,000 hectares of English countryside by 2030 under government plans to support the recovery of nature.
The prime minister will make the pledge at a virtual United Nations event on Monday, where he will warn that countries must act to reverse biodiversity loss.
National parks, areas of outstanding natural beauty and other protected areas make up 26% of land in England.
Mr Johnson will promise that the government will increase the amount of protected land in the UK from 26% at present to 30% by 2030.
The environment is a devolved matter, but the government has said it will work with Scotland, Wales and Northern Ireland, as well as landowners, to increase the size of protected land across the UK.
Mr Johnson will say countries must turn "words into action" and "agree ambitious goals and binding targets".
"We cannot afford dither and delay because biodiversity loss is happening today and it is happening at a frightening rate," he will say.
"Left unchecked, the consequences will be catastrophic for us all. Extinction is forever – so our action must be immediate.”
Environmental organisations welcomed the commitment but called for government to invest in existing protected sites, and to put the new pledge into law.
Campaigners say if the UK is really taking nature seriously it must avoid trade deals that damage wildlife. They also added that the government need to clamp down on imports of food that have caused environmental destruction overseas - such as beef farming in the Amazon.
WWF-UK chief executive Tanya Steele said: “This announcement is a welcome step, but it must be backed up by urgent ambition, including strong legislation to avoid damaging trade deals and to stop the food we eat from destroying the environment here and abroad."
Martin Harper, the RSPB's director of global conservation, said the 30% commitment could be a "huge step towards addressing the crisis our wildlife is facing".
"However, targets on paper won't be enough," he said.
Mr Harper said the pledge must be put into domestic law "as part of a suite of goals to restore the abundance and diversity of our wildlife, in every country in the UK".
Craig Bennett, chief executive of The Wildlife Trusts, said it was a "good start" but "a much greater level of urgent action" was needed to put nature into recovery, including rescuing wildlife sites currently in decline.