The Campaign to Protect Rural England has warned that Natural England, the new central government body for the countryside, could be "too poor and too weak" to continue funding voluntary organisations.
A statement from the charity described Natural England as a "starved and toothless watchdog" that faces "huge challenges".
The watchdog, launched this month, brings together English Nature, the Countryside Agency and the Rural Development Service. It aims to maintain and promote a natural environment and create a secure environmental future.
However, Tom Oliver, head of rural policy at the CPRE, said "savage cuts" to Natural England's core funding, compared with the funding of its predecessors, meant the organisation was "weakened even as it embarks on its crucial work".
Oliver fears that cuts to funding will stop Natural England from continuing the small-scale contracts with third sector organisations that were previously offered by the Department for Environment, Food and Rural Affairs.
The RSPB, which received a grant of almost £500,000 a year from English Nature for its Action for Birds in England programme, said that although Natural England claims it will meet continuing contracts, funds have yet to be announced.
"Until we see what resources are available, it is appropriate to worry and to bring to ministers' attention the need for Natural England to meet its statutory requirements," said a spokesman for the RSPB.
Hilary Allison, policy director at the Woodland Trust, said the potential for Natural England to be an effective force for landscape and wildlife was "quite strong". But she added: "We will be watching it carefully to see how well it matches expectations."