Plans to water down protections for national parks such as Dartmoor and the Lake District in a “kneejerk bid for growth” will be devastating for nature, more than 170 organisations have told the prime minister.

The Treasury is understood to be pushing for a weakening of protections for England’s national parks and national landscapes in changes to planning law to make it easier for developers to build houses and infrastructure projects.

National parks were created by Clement Attlee’s Labour government in the 1949 National Parks and Access to the Countryside Act. Keir Starmer’s Labour government has declared the protection and enhancement of national parks to be vital to its mission to restore nature and boost the nation’s health through access to green spaces. Ministers celebrated the parks last year on the 75th anniversary of their creation.

But a suggested change being promoted by the chancellor, Rachel Reeves, to the planning bill would remove a key duty on councils and other public bodies to protect and enhance the nature and wildlife within national parks, seriously weakening the protections for these much-loved areas, charities say.

It is understood discussions on the changes are ongoing within governmentand that ministers have until Monday to come forward with amendments to the planning bill. However, a government spokesperson told the Guardian that it was untrue that these changes were being discussed.

Rose O’Neill, the chief executive of the Campaign for National Parks, said there was no evidence that the duty to enhance the parks was stopping growth. She said the last-minute proposal was a kneejerk reaction backed by no evidence.

“Keir Starmer has talked so much about his love of national parks, and the Lake District, so we are appealing to him to stop this,” she said. “We don’t think removing this duty will do anything to boost growth; in fact, it will slow down development. There has been no public consultation on what amounts to a serious weakening of protections for national parks and landscapes. It was only in December last year that this government was celebrating them on their 75th anniversary as part of Labour’s postwar legacy.”

Currently, public bodies have to “seek to further” the statutory aims of national parks, which are to conserve and enhance their natural beauty, wildlife and cultural heritage, early in any planning process and to mitigate any damage. The duty, contained within the Levelling-up and Regeneration Act 2023, was supported by Labour in opposition.

Any weakening of protections is likely to make it easier to build roads and other major infrastructure within national parks, including Dartmoor, the New Forest and the Lake District, the organisations say.

The Guardian recently revealed that the UK ranked as one of the worst countries in Europe in terms of sacrificing green space to development. Thousands of English nature sites are at risk under Labour’s planning proposals.

In a letter to Starmer on Tuesday, the 170-plus groups – including the RSPB, the Wildlife Trusts, the Rivers Trust and the National Trust – said national parks and landscapes were enjoyed by 245 million visitors a year, generating £36bn for the economy. They appealed to Starmer to step in and ensure there is no rollback of protections, citing his declared love for the Lake District.

The letter said: “[National parks] were established after the second world war as part of national renewal, driven by a Labour government who understood the value of such places for the nation and that the simple joy of beautiful landscapes was part of what makes a good life, which should be the right of every citizen whether they live in a town, city or the countryside.

“Now we understand some in your government intend to significantly weaken protections by removing the protected landscapes duty, a move at odds with this proud history and values.”

Reeves has been looking for ways to strengthen the planning bill, which is currently in the House of Lords, the Guardian has revealed. She is set to announce her support for changes to the bill in the hope that it will pass the Lords and receive royal assent before her budget next month. She hopes that the building of 1.5m homes and key infrastructure projects will fill £3bn of an estimated £30bn hole in the finances.

But in the letter delivered on Tuesday, the nature organisations said the health of the environment underpinned the health of the economy.

“Skewing this balance will have devastating consequences – and future generations will inherit the mess,” it said. “Clement Attlee’s postwar government understood this: it is why they created national parks and national landscapes … protecting landscapes alongside rapid housebuilding.”

A government spokesperson told the Guardian: “These claims are untrue. We will deliver the infrastructure and 1.5 million homes the country needs while preserving the natural beauty of our Protected Landscapes.”