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 was cut by a quarter from 2011 to 2016, according to an investigation by the agency.
The government is committed to raising parks' budgets between now and 2020.
However, funding in 2020 will be as much as a fifth below 2010 levels, said the study, even before inflation is taken into account.
Among the parks affected were 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 found that once inflation was 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) pledged to increase the direct grant for National Parks by 1.72% a year for most parks up to 2020.
The figures exclude South Downs National Park which did not become fully operational until April 2011.
The park saw funding fall from £11.4m in 2011-12 to £9.8m in the last financial year.
The Yorkshire Dales will see 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 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 said the charity welcomed the government's desire for more people to benefit from National Parks.
"If we want the parks to inspire current and future generations we need to make sure they receive the resources necessary for them to be maintained and, ideally, enhanced," she said.
A Defra spokeswoman said: "National Parks are treasured landscapes and an important part of our country's identity, attracting 90 million visitors and generating £4bn a year.
"We are committed to helping them thrive, which is why we have protected their budgets to 2020, committing over £350m for English National Parks, AONBs (Areas of Outstanding Natural Beauty) and public forests."
National Parks in England are funded by central government.
They also get some money from other sources such as the European Union and earning money themselves.