National Parks in England have lost millions of pounds of government funding in the past five years, says the Press Association news agency.
Their funding had been cut by a quarter from 2011 to 2016, according to an investigation by the agency.
The government has been committed to arrange parks' budgets between now and 2020.
According to a study, funding in 2020 will be as much as a fifth below 2010 levels, even before inflation has been taken into account.
Among the parks affected had been the Peak District, with annual funding cut from £8.3m to £6.3m, and the Lake District from £6.9m to £5.2m.
The investigation has found that once inflation had been taken into account cuts over the five years were even more severe, at 40% in real terms.
In 2015, the Department for Environment, Food and Rural Affairs (Defra) had pledged to increase the direct grant for National Parks by 1.72% a year for most parks up to 2020.
The figures excluded South Downs National Park which has not become fully operational until April 2011.
The park saw that the funding has fallen from £11.4m in 2011-12 to £9.8m in the last financial year.
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The Yorkshire Dales observes a bigger boost in its grant in the next two years as a result of the extension of the park area by almost a quarter in 2016.
The Campaign for National Parks has said that since 2010, the Norfolk Broads had closed three out of six information centres, Dartmoor had reduced staff by 35% and various parks had cut bus services transporting people to the attractions.
Chief executive Fiona Howie informed that the charity has welcomed the government's desire for more people to benefit from National Parks.
She said, "If we want the parks to inspire current and future generations we have to make sure that they have received the resources necessary for them to be maintained and, ideally, enhanced."
A Defra spokeswoman told, "National Parks are treasured landscapes and an important part of our country's identity, attracting 90 million visitors and generating £4bn a year.”
By Prakriti Neogi
