Affordable homes and jobs are needed on Dartmoor and Exmoor to prevent the areas from becoming "retirement zones" according to Britain's biggest landowners' organisation
The Country Land and Business Association said it was important to attract young families to the National Parks if they were to thrive. CLA chiefs said access to services was also required to attract young families.
Without action the CLA said it fears the parks risk turning into permanent retirement and dormitory zones rather than the living and working communities needed to keep their "unique characteristics."
The warning follows an analysis of the 2011 Census which highlights a decline in population in the national parks.
John Mortimer, CLA South West director, said it was a "real challenge" to keep young families living and working in often remote rural communities. He said: "Retirement villages will not generate the economic activity essential to the maintenance and management of the countryside.
"Future planning policies in national parks must recognise the need for affordable housing, for jobs and accessible services. The days when people were employed exclusively on the land and in the village bakeries and blacksmiths have long gone.
"But planning policies need to adapt and to become more flexible if we are to enable the communities living within the national parks to become genuinely sustainable.
"The unique landscapes of the South West national parks were, he said, inextricably linked to their economic viability.
"Looking after the landscape, the habitat and biodiversity costs money – which is why the CLA has long called for a balanced approach to be taken which takes all three pillars of sustainable development into consideration, economic, social and environmental.
"If agriculture, diversification, housing, jobs and tourism are stifled or put at risk, then the landscape which depends upon them – and which the public so loves – will degrade."
The CLA has said the economic challenge facing national parks can only be solved by taking a balanced approach to the economic, social and environmental pillars of sustainable development.
Housing experts backed the CLA's view. Richard Copus, spokesman for the National Association of Estate Agents, said there was a need for affordable homes for local people.
He said: "Twenty years ago any talk of building new homes on the national parks would have been shouted down. There will always be an element of nimbyism – but there's been a definite sea-change because wages have not kept up with house prices."