A dozen organisations have raised concerns over privatisation plans they fear will restrict access to the countryside.
The Ramblers, the Open Spaces Society and the British Mountaineering Council are among groups opposing parts of the Public Bodies Bill, which paves the way to dispose of the public forest estate to private companies, communities and charities.
The Government is consulting on controversial proposals to offload up to 100 per cent of England's 258,000-hectare public forest estate, currently managed by the Forestry Commission, over the next 10 years.
The proposals include a 250m sale of leaseholds for commercially valuable forests to timber companies, measures to allow communities, charities and even local authorities to buy or lease woods, and plans to transfer well-known "heritage" woods such as the New Forest into the hands of charities.
The 12 outdoor and nature organisations are warning that current commitments to look after the landscape, wildlife and public access on foot, horseback and bike may be undermined.
In a joint statement, they said elements of the Bill, which is currently in the House of Lords, would give Government "wide-ranging and unfettered powers to alter the nature and functions of bodies which hold land for the public benefit".
The groups said they were concerned about possible changes to the Forestry Commission and National Park Authorities, which have a statutory commitment to protect the landscape and wildlife and provide good public access.