The report, published by the Campaign for National Parks, Campaign to Protect Rural England (CPRE) and the National Trust, examined national park authorities' interpretation of the major development test, which it says makes clear that planning permission should be refused for major developments in national parks except in exceptional circumstances and where it can be demonstrated they are in the public interest.
The report, based on research conducted by Sheffield Hallam University, found that the test is generally "fit for purpose" and well supported by the park authorities. But it also found that there is "considerable variation" between the authorities in the definition they use for major development and the way in which they implement the test, both in policy and in practice.
"This can lead to confusion and the potential for inadvertent policy shifts as park authorities apply different degrees of local context or national significance within their definitions," the report said.
The report said that the precise wording in the test has been amended over time in response to government planning policy changes. But it found little evidence to suggest that changes to major development policy have had any significant impact on local decisions in the national parks.
"Instead, decisions appear to reflect central government’s agenda at any particular time and the continuing challenge of supporting national park purposes whilst enabling local economic development," the report said.
The report urges national park authorities to "ensure their local plans are clear about how the major development test should be applied in relation to the special qualities of the national park in order to help reinforce and support local decision-making".
It also recommends that Natural England should take a "more active role" in ensuring that national parks are effectively protected from major development.
"This should include producing an annual update setting out how the major development test is being implemented and providing guidance or training for park authorities to address any issues identified," the report said.
Emma Marrington, senior rural policy campaigner at CPRE, said: "National parks should have the highest level of planning protection, but in practice this can be overridden in the interests of short term economic gain.
"Our research has shown that national planning guidance should be improved so that when developments are considered, public bodies and developers give due regard to the importance of national parks."
In a statement, the Department for the Environment, Food and Rural Affairs, said: "National Parks are treasured natural landscapes and part of our country’s identity, which is why we have afforded them the highest level of protection.
"Planning powers lie with National Park Authorities, and they strike a balance between the economic benefits of development and protection of these beautiful natural places."
National Parks: Planning for the Future is available here.