Britain will need to build on protected areas of the countryside if it is to meet a target of 3 million new homes by 2020, a study released yesterday argues.

As many as 1.8 million new homes will have to be built on undeveloped agricultural or natural land, the Social Market Foundation, a research organization, said in a report. Unused industrial land can only accommodate 1.2 million units, it said.

Prime Minister Brown has left the door open to redrawing greenbelt boundaries established to stop urban sprawl, putting his government on collision course with local communities and environmental campaigners. Sixty percent of new homes will be built on previously developed land under government plans.

“The U.K. faces tough choices in meeting its housing needs,” the director of Social Market Foundation, Ann Rossiter, said. “There is no easy answer and wherever we build new houses involves trade offs. It is time for a sensible debate.”

House prices in Britain have tripled since the Labour government took office in 1997, as supply failed to keep pace with a rising population.

Homebuilding stagnated at 148,000 new units a year on average between 1989 and 2005, down from a peak of 425,000 in 1968, government figures show. About 180,000 homes were completed last year, and the government wants to increase that number to about 240,000 by 2016. Housing Minister Yvette Cooper said July 23 that the government proposes building 45 new towns.

The Social Market Foundation said the target of 3 million homes is the minimum needed to ease pressure on the housing market. “Simply letting the market rip in areas where it would like to go, very often in greenbelt areas, won’t necessarily put development in the places that will do the most good for everybody in town and country alike,” a partner at Green Balance, which advises on environmental issues, Richard Bate, told BBC radio’s Today program.

The government said it will do everything it can to protect the countryside.

“We believe it is possible to build the homes future generations need whilst protecting the environment and green spaces,” the minister of Communities, Kay Andrews, said in an e-mailed statement. “Our clear priority for development will remain brownfield land.”

At present, 74% of new housing is being built on brownfield sites compared with 57% in 1997, she said.

Any encroachment into the greenbelt may create a dilemma for the main opposition Conservative Party. While leader David Cameron has thrown his weight behind tackling the housing shortage, his party draws much of its support from rural areas and many grassroots activists are opposed to mass housebuilding.

“The greenbelt has served England well for the last half century, protecting against urban sprawl and unsustainable development,” the Conservative planning spokesman, Eric Pickles, said in a statement.

“We need to build more homes and regenerate rundown communities; yet greenbelt protection must remain.”