'If this means vast swathes of valuable forest being sold to developers, it will be an unforgiveable act', says Caroline Lucas

Many of England's best-loved forests and woodlands may be sold to large landowners, housing developers and international power companies in what could be the UK's greatest change of land ownership since the second world war.

Caroline Spelman, the environment secretary, is expected to announce a new strategy later this week that will lay the foundations for more than 150,000 hectares of forest and other land owned by the state in England to be sold within three years.

Tonight, conservationists and opposition parties as well as landowners warned that the land sale would be a costly disaster unless stringent safeguards were put in place.

"If this means vast swathes of valuable forest being sold to private developers, it will be an unforgiveable act of environmental vandalism. Rather than asset-stripping our natural heritage, government should be preserving public access to it, and fostering its role in combating climate change and enhancing biodiversity," said Green MP Caroline Lucas.

The part-privatisation of the biggest landowner in England could raise around £250m at present land values. But charities and ecologists warned tonight that it could set back countryside protection and the restoration of ancient woodlands by many years if industry is allowed to cherry-pick the most profitable land and cut off funds now used for conservation and afforestation.

In addition landowners warned that the land could be snapped up by industrialists with no concern for the environment or landscape value. "I have no doubt that this is something our members would be interested in. What we would be concerned about is if the land is put on the market all at the same time. This would enable industrial landowners to buy them all up and aggressively control the market", said Mike Seville, forestry and woodland advisor for the Country Landowners' Association.

"This sale is likely raise a tiny amount but could do immeasurable damage and cost the nation dearly. This land will not be sold for anything like its real environmental and social value. There is no way the private sector is going to provide the same level of care [as the commission]," said Paul Hetherington, spokesman for the Woodland Trust.

Mark Avery, conservation director at the RSPB, said: "The future ownership and management of land that has high public value should be carefully considered. The proposed land sales are driven by the need to generate quick cash, but they must not be at the expense of protecting our natural capital, which is irreplaceable."

Whitehall sources confirmed that privatisation would be an element of the plan, which is due to be announced within days, but said it will not be a complete sell-off of forestry land.

Mary Creagh, the shadow environment secretary, said: We are concerned developers will cherry pick the most profitable land and we will see huge pressure for development in sensitive places. The environment is going to pay a high price for its settlement in the recent spending review."

Both the Thatcher and Major governments tried to privatise the Forestry Commission in the 1980s and 1990s but failed following intense pressure from conservation groups and lack of interest by industry.

Since then, however, land has become more valuable, not just for timber but for providing "environmental services" such as flood control, climate change measures and amenity.

In England the commission is subsidised by £30m a year, but generates an additional £63m a year in income. A government economic study released earlier this year calculated that it provides £2,100 in value per hectare per year if benefits such as erosion protection, pollution absorption, carbon sequestration, health provision are included.