Plans to sell off as much as 150,000 hectares of forest and woodland in England in the biggest sale of public land for nearly 60 years were today confirmed by the government in a letter sent to all MPs.

"[Our] intention is to fundamentally reform the public forestry estate, with diminishing public ownership and a greater role for private and civil society partners," said a statement on the Department for Environment, Food and Rural Affairs (Defra) website.

"This will be a new approach to ownership and management of woodlands and forests, with a reducing role for the state and a growing role for the private sector and civil society. We are committed to shifting the balance of power from 'big government' to 'big society' by giving individuals, businesses, civil society organisations and local authorities a much bigger role in protecting and enhancing the natural environment and a much bigger say about our priorities for it."

No details were given on which tracts of land would be sold but the government insisted that conservation would not be compromised and this would not lead to a mass felling of woodlands.

"Full measures will remain in place to preserve the public benefits of woods and forests under any new ownership arrangements. Tree felling is controlled through the licensing system managed by the Forestry Commission, public rights of way and access will be unaffected, statutory protection for wildlife will remain in force and there will be grant incentives for new planting that can be applied for."

The measures, which were leaked this week, will be contained in the public bodies bill. They have already provoked a storm of controversy with opposition from the public and unions but guarded enthusiasm from many charities and conservation groups who expect to benefit from buying or managing the woodland that will become available.

"This will fail on all levels," said Malcolm Currie, a negotiator for the union Prospect. "Three thousand employees' jobs and futures will be under threat as the land is pulled from under their feet and thousands of jobs in the private sector – in planting, growing, felling, hauling and milling wood – will be placed in peril. It's a prime example of the absurdity of the government's argument that the private sector will compensate for job losses in the public sector."

A spokesperson for the Woodland Trust said: "If revenue from these sales does not find its way back into Forestry Commission income streams, which looks likely, it could seriously threaten the commission's ability to support the future planting of new native woodland, which is a major priority for us and for government response to the climate change agenda, as well as into restoration of planted ancient woods."