The National Trust’s intervention will ignite the current row over Gordon Brown’s plans to build three million new homes by 2020.

The Government’s own advisers argued last week that the plans were not ambitious enough and called for 3.25 million extra houses. They claimed that unless more construction went ahead in the South, South West and South East, house prices in all three areas would become unaffordable for first-time buyers.

Rural campaigners and environmentalists, however, have given warning that the houses and related infrastructure will ruin large swaths of the countryside, put huge pressure on water supplies and increase carbon emissions. They have become increasingly concerned that the Government will change its current green belt policy and sanction urban sprawl around Britain's towns and cities.

Mr Brown pledged in the summer that the present protection would remain, but Kate Barker and Natural England, both advisers to the Government on planning, have called for a review of the green belt in the light of spiralling demands for new homes.

The Campaign to Protect Rural England has been at the forefront of efforts to stop housing developments on greenfield and green belt land, particularly in the South and South East, where housing shortages are most acute. It claims that Ms Barker’s suggestion and the recent intervention by Natural England is already prompting speculators to earmark green belt land for development.

Under current guidelines 60 per cent of development has to be on brownfield sites and 40 per cent on greenfield and green belt. The Government says that at least 74 per cent of building is on brownfield land, well above guidance, a figure that has risen consistently over the past ten years.

The regional assemblies in the South and East of England have also been resisting ministers’ proposals for extra homes and delaying any construction. The assemblies, councillors, trade unions and business representatives argue that the plans are opposed by residents and not backed by enough funding for schools, transport, hospitals and other infrastructure.

Ministers have been so frustrated, however, by the constant attempts by assemblies to revise plans downwards that they have announced their abolition by 2009. Last week Yvette Cooper, the Housing Minister, also gave councils the green light to review existing regional plans and announced new financial incentives to get them to speed up housing developments.

On the other flank they are being pressed by the National Housing Federation, Shelter and the Town and Country Planning Assocation over the desperate need for social and private homes for first-time buyers, migrants and a growing elderly population.

When Mr Brown became Prime Minister he made housing one of his top priorities. He raised the annual housebuilding target from 200,000 to 240,000 a year by 2016 with the aim of constructing three million homes by 2020. Last week the National Housing and Planning Advice Unit (NHPAU) called for 270,000 a year to be built by 2016, boosting the overall total to 3.25 million by 2020.

The analysis also made clear that the type, timing and location of housebuilding would affect prices. Building levels would have to be increased first in the least-affordable regions to stabilise prices and affordability, it says.

Stephen Nickell, chairman of the NHPAU, said: “If we fail to act, then a generation of buyers will be unable to get a foothold on the housing ladder, not just in London but across large swaths of England, and current home-owners will not be able to move on to bigger and better homes.”

Last night the Government dismissed the comments by Sir William Proby, the chairman of the National Trust. “This is inaccurate and misunderstands the planning protections that are in place to safeguard the countryside and open green space,” a Communities Department spokesman said. “The priority put on brownfield development has substantially increased protection for the countryside, with 74 per cent of all new housing currently being built on recycled land, up from 57 per cent in 1997.

“Over the same period, we have seen the amount of green belt land increase by 64 thousand acres. For the sake of first-time buyers and families on council waiting lists, there is a need to build more homes but we believe it is possible to do so whilst protecting the environment and green spaces.”

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