The National Trust is to put itself on a collision course with the Government over Gordon Brown’s plans to start the biggest house-building programme in 30 years.

The trust, one of Britain’s largest landowners with 700 miles of coastline and 250,000 hectares of countryside, has threatened to buy up greenfield land earmarked for development to stop new housing. It is also preparing to intervene in planning inquiries and challenge new developments, even if it has no direct involvement with the land under threat.

The Prime Minister has pledged to build three million new homes by the end of the next decade, with the annual housebuilding target to be increased to 240,000 homes a year. The last time that figure was reached was in 1979, when council houses were still being built.

But in a direct attack on the Government, the National Trust has accused ministers of destroying vital green spaces by pressing ahead with housebuilding, airport expansion and new industrial sites. It has already shown itself a determined opponent of the Government over the plans to expand Stansted airport, which it claims will destroy the tranquillity and oasis of Hatfield Forest — which it owns. The Trust was represented at the recent planning inquiry.

Sir William Proby, its chairman, is to use the trust’s annual meeting to give notice that Europe’s biggest membership organisation — with 3.5 million members — will not sit back and allow the desecration of the green belt or other open spaces.

The trust is exasperated because ministers and senior civil servants appear ready to develop green fields without considering the quality of life offered to local communities by the presence of open spaces.

In his speech today Sir William will say: “Inch by inch, year by year, the redeeming, restorative qualities [of the countryside] are being eroded. We are not against progress. But the sheer scale of what is being contemplated goes way beyond this.”

In an interview with The Times Sir William made clear that if necessary the trust – which has an annual income of £350 million – would buy up land or development rights on land to save green fields.

Another ploy may be to encourage property owners to covenant land to the trust so that it can take on responsibility for protection of these sites.

“In the planning process I am not sure the value of the spiritual side and the beauty of landscape and its importance on the quality of life rather than the financial gain is taken into account,” he said. “It should do that. But I feel the process is being driven by economic arguments from a top-down planning process.”

It is the the scale of the proposed development, however, that disturbed him most. “Development is being pushed through. The process is being driven by narrow economic objectives and once we’ve lost these green spaces they have gone forever and there is no turning back.”

He said that Whitehall had turned a deaf ear to its representations, that there should be a new method to evaluate the landscape and to take local views into account. “We haven’t seen much evidence of government responding to what I am talking about. In fact, we feel the opposite and that the juggernaut is proceeding.”

He was also angry at the vilification of local people who protested against new development. “Nimbys (Not In My Back Yards) have a bad name, especially in terms of the landscape. Sometimes people disparage local opposition, but it is misguided.

“It is legitimate for people to be concerned about their local environment. They should have a chance for their voice to be heard. Too many genuinely public-spirited citizens are being unjustly tarred with the Nimby brush.”

Sir William admitted that he was not convinced about plans for new eco-towns and green living. Some loss of the countryside was inevitable but there should be more use of brown-field sites and greater local consultation. In particular, the trust’s concerns are for the South East and the East of England, the areas most under pressure for new building.

A move to protect open spaces is a return to the original ethos of the trust, which was inspired in part by Octavia Hill’s bitter but unsuccessful campaign in the 19th century to save fields around Swiss Cottage, in northwest London, as a public amenity.

But Sir William said that he was also prepared for the trust to oppose development plans for land where it had no direct involvement. “It will depend on the area, where the landscape is and the amount of its importance. But it is legitimate for us to be concerned with all landscape.

“The Government – any government – is now at least forewarned. We are watching. And we are watching with the interests of future unborn generations in mind.”

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