Since the 1940s, the Metropolitan green belt has sat like an impregnable buffer around London, protecting property prices with its promise of permanent open fields safe from developers.

But are the cracks beginning to appear? The Council for the Protection of Rural England (CPRE) warns of "death by a thousand cuts" in the face of the relentless demand for new homes.

It notes that while the government professes to support the green-belt concept, in the past four years it has waved through 119 out of a total of 251 development proposals in green belts across England, eating up 1,300 hectares in the process.

Any official government changes to planning guidelines will appear in about two years' time, in the revision of Planning Policy Guidance (PPG) notes to local authorities.

Some, however, are already suggesting the previously unsayable. In a recent internal discussion document, the Royal Town Planning Institute called for the green belt to be "modernised".

According to David Barraclough, RTPI planning policy officer and author of the report: "It should not be sacrosanct and unmovable when all other aspects of the planning system are being reviewed by New Labour."