However, he said that changes announced in the National Planning and Policy Framework (NPPF) would mean that the impact of the grey belt would be "at best, marginal".

The changes include a requirement for councils to review green belt boundaries and suggest changes to the boundaries if they cannot meet their housing targets.

Under the new rules, building on green belt land will be be allowed if other options have been exhausted, the government has said.

"It is difficult to see what the grey belt regime will add in terms of the ability to build on the green belt in light of this requirement," Lord Moylan said.

In the course of taking evidence the committee heard dramatically differing estimates of how many dwellings could be built on grey belt land.

Homebuilders Barratt Redrow suggested the number could be as low as 50,000, while LandTech, a software company, put the figure at four million.

Responding to the inquiry's conclusions, Fergus Charlton, a planning partner at firm Michelmores, said: "Whether or not the grey belt policies will play an effective role in achieving the government's difficult to reach 1.5 million new homes target will become clearer as the policies begin to play a defining role in plan making.

"In the realm of decision-taking they are already playing a key role in the outcome of appeals where green belt land is considered by inspectors to be grey belt.

"We have seen more than a handful of successful appeals in the eight weeks since the grey belt concept was confirmed in the NPPF."