What is the ‘grey belt’ and how many homes could Labour build?

Labour floated the idea of creating the grey belt long before the election.

When Sir Keir Starmer mentioned it over a year ago, then prime minister Rishi Sunak ruled it out, external as he “wanted to make sure our green spaces are protected”.

Building on the green belt is controversial.

However, Ms Townsend of the Centre for London says attitudes might be shifting, pointing to polling , externalshowing half of Londoners supported building strategically on low-quality areas of the green belt, with only 19% against it.

Others disagree. Countryside charity CPRE Oxfordshire, for example, says land is “already under enormous pressure”.

Its director Helen Marshall says: “Ten years ago we commissioned research which showed that over 70% of Oxfordshire wanted the green belt to remain undeveloped.

“Ten years later support for the green belt has increased to over 80%.”

However, Mr Hart of Knight Frank says that while the green belt used to be seen as a “don’t go zone”, perceptions are shifting.

“It was conceived a long long time ago and the world is a very different place and we’re now reviewing whether it’s fit for purpose in the modern world.”

Additional reporting by Gerry Georgieva