has been accused of "sacrificing" to "appease his Labour paymasters" following plans to build on the precious land in a bid to tackle the housing crisis.
The capital's mayor will announce plans to "actively explore" building on some of London's green belt on Friday in a bid to fix the capital's housing crisis.
Susan Hall AM, Leader of the City Hall Conservatives, said: "Once again, Sadiq Khan has shown his true colours by dancing to the tune the Labour Government sets. This is a Mayor who will sacrifice London's precious green spaces to appease his Labour paymasters. Londoners elected him on a promise to protect our environment, but it's now clear those promises weren't worth the recycled paper they were printed on."
The Mayor of London is expected to use a speech in Greenwich to argue that "the status quo is wrong, out-of-date and simply unsustainable", with a generation of Londoners unable to rent or buy a home.
Kevin Hollinrake MP, Shadow Secretary of State for Levelling Up, Housing and Communities, said: "The public has been completely conned. Starmer and Rayner dressed up their so-called 'Grey Belt' plan as a few disused garage forecourts - but in reality, it's a green light for Sadiq Khan to concrete over thousands of acres of protected Green Belt land to cover up his appalling housing record.
"Year after year, Sadiq Khan has failed to deliver the homes London needs. Now, instead of taking responsibility, he's trying to bulldoze his way out of failure by sending the bulldozers onto our green fields.
"Labour's approach is always the same: tear up the rules, silence local communities, and sweep aside long-standing protections. There are thousands of brownfield sites in London crying out for regeneration- but Labour would rather sacrifice green space than do the hard work of proper planning.
"This is the consequence of reducing London housing targets and rewarding failure. Labour is out of touch, out of ideas, and far too willing to sell out Britain's green spaces."
Under the proposals, London's planners will look to release some green belt land for housebuilding, but with conditions on affordable housing, energy efficiency, transport links and promoting access to green spaces such as parks.
The announcement marks a change in stance for Sir Sadiq, who has previously argued against releasing green belt land while there are still previously developed brownfield sites available.
While the Mayor is expected to continue prioritising brownfield land, he is expected to tell his audience on Friday: "We have to be honest with Londoners that this alone will not be enough to meet our needs."
Sir Sadiq will launche a consultation on the next version of the London Plan, which will set out his vision for development in the capital over the next two decades.
London has a new target of building 88,000 new homes a year, but the current plan, which expects all housing supply to come from brownfield sites, is delivering only around 40,000 a year.
Meanwhile, rents in London rose by 11.5% last year and councils are spending £4 million a day on temporary accommodation amid increasing homelessness.
City Hall has already begun a review of green belt land in line with the Government's policy of building more on low-quality parts of the green belt, so-called "grey belt" land.
But Friday's announcement is expected to go further, investigating other possibilities for securing enough land to meet London's housing needs.
Sir Sadiq will say: "The green belt can often be low-quality land, poorly maintained and rarely enjoyed by Londoners. Only around 13% is made up of parks and areas that the public can access.
"So given the quality of parts of London's green belt and the extent of the housing crisis, I believe the status quo is wrong, out-of-date and simply unsustainable.
"Development on carefully chosen parts of the green belt - done in the right way - would allow us to unlock hundreds of thousands of good-quality new homes for Londoners. This would not only go a long way to ending the housing crisis but provide a huge boost to our economy."
The Government has welcomed the move, with Deputy Prime Minister Angela Rayner calling it a "bold proposal" that "rises to the challenge of tackling London's housing crisis".
Claire Holland, leader of Lambeth Council and chairwoman of London Councils, also backed the proposal, saying: "London is grappling with the worst housing pressures in the country and there is a desperate need to build new homes, particularly more affordable homes.
"Boroughs are resolutely pro-housebuilding and are committed to working with the Mayor and the Government to boost housing delivery across the capital."
Ben Twomey, chief executive of campaign group Generation Rent, said: "When housing costs are forcing too many of us into poverty and homelessness, it is right that the Mayor sets out his ambition to build the homes we need, including in parts of the green belt where appropriate."
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