Legislation which currently governs the treatment of "ancient forests" such as the Forest of Dean and Sherwood Forest is likely to be changed giving private firms the right to cut down trees.
Laws governing Britain's forests were included in the Magna Carta of 1215, and some date back even earlier.
Conservation groups last night called on ministers to ensure that the public could still enjoy the landscape after the disposal, which will see some woodland areas given to community groups or charitable organisations.
However, large amounts of forests will be sold as the Department for the Environment Food and Rural Affairs (Defra) seeks to make massive budget savings as demanded in last week's Spending Review.
Whitehall sources said about a third of the land to be disposed of would be transferred to other ownership before the end of the period covered by the Spending Review, between 2011 and 2015, with the rest expected to go by 2020.
A source close to the department said: "We are looking to energise our forests by bringing in fresh ideas and investment, and by putting conservation in the hands of local communities."
Unions vowed to fight the planned sell-off. Defra was one of the worst-hit Whitehall departments under the Spending Review, with Ms Spelman losing around 30 per cent of her current £2.9 billion annual budget by 2015.
The Forestry Commission, whose estate was valued in the 1990s at £2.5 billion, was a quango which was initially thought to be facing the axe as ministers drew up a list of arms-length bodies to be culled.
However, when the final list was published earlier this month it was officially earmarked: "Retain and substantially reform – details of reform will be set out by Defra later in the autumn as part of the Government's strategic approach to forestry in England."
A spokesman for the National Trust said: "Potentially this is an opportunity. It would depend on which 50 per cent of land they sold off, if it is valuable in terms of nature, conservation and landscape, or of high commercial value in terms of logging.
"We will take a fairly pragmatic approach and look at each sale on a case by case basis, making sure the land goes to the appropriate organisations for the right sites, making sure the public can continue to enjoy the land."
Mark Avery, conservation director for the Royal Society for the Protection of Birds (RSPB) said: "You can understand why this Government would think 'why does the state need to be in charge of growing trees', because there are lots of people who make a living from growing trees.
"But the Forestry Commission does more than just grow trees. A lot of the work is about looking after nature and landscapes."
"We would be quite relaxed about the idea of some sales, but would be unrelaxed if the wrong bits were up for sale like the New Forest, Forest of Dean or Sherwood Forest, which are incredibly valuable for wildlife and shouldn't be sold off.
"We would look very carefully at what was planned. It would be possible to sell 50 per cent if it was done in the right way."
A Defra spokesman said: "Details of the Government's strategic approach to forestry will be set out later in the autumn.
"We will ensure our forests continue to play a full role in our efforts to combat climate change, protect the environment and enhance biodiversity, provide green space for access and recreation, alongside seeking opportunities to support modernisation and growth in the forestry sector."
Allan MacKenzie, secretary of the Forestry Commission Trade Unions, said: "We will oppose any land sale. Once we've sold it, it never comes back.
"Once it is sold restrictions are placed on the land which means the public don't get the same access to the land and facilities that are provided by the public forest estate.
"The current system means a vast amount of people can enjoy forests and feel ownership of them. It is an integral part of society."
In 1992 John Major's Conservative government – also looking to save money in a recession – drew up plans to privatise the Forestry Commission's giant estate, which ranges from huge conifer plantations to small neighbourhood woodlands.
John Gummer, then the Agriculture Minister, wrote to cabinet colleagues saying that he 'wanted to raise money and get the forest estate out of the private sector'. Mr Major backed the sell- off which, it was hoped, would raise £1 billion.
However it was later abandoned following a study by a group of senior civil servants, amid widespread public opposition.
Sale in small lots - 1 acre upwards - allow the people to 're-own' their own lands finally
