Conservationists fear parts of Devon and Cornwall's most beautiful and important areas of countryside could be privatised.
Some national nature reserves (NNRs) could be sold to charities and businesses as part of the government's spending cuts.
If the move goes ahead, it could affect any of the 12 reserves in the region - three in Cornwall and nine in Devon.
Natural England said no decision had yet been made.
It said all government departments were having to make significant savings.
The Department for Environment, Food and Rural Affairs (Defra) has to save £162m
As part of those savings, Natural England said its budgeted income had been reduced by £7.4m.
But it said spending decisions affecting the next five years were still the subject of debate.
"As such, it would be premature to comment on any implications of decisions that have yet to be made," a statement added.
Dartmoor Action Group described the privatisation plans as "environmental asset stripping of the worst possible kind".
"It's going to throw away 50 years of very hard work building up these nature reserves," spokesman John Bainbridge told BBC News.
"We don't know what the future's going to be.
"We don't know if the people taking over are going to be able to fund them properly or whether they'll allow public access.
NNRs were established to protect sensitive areas, but most now offer educational opportunities to the public to experience England's natural heritage.
If the nature reserves are sold, the land would retain its legal protection, but the maintenance would pass from Natural England to the charities or businesses which had purchased them.
Conservation groups have said they intend to begin campaigning to fight the privatisation proposals.