Sir Sadiq Khan is to announce plans to “actively explore” building on some of London’s green belt in a bid to fix the capital’s housing crisis.

The mayor of London is to argue that current rules prohibiting development on the green belt are “wrong, out-of-date and simply unsustainable”, with many Londoners unable to rent or buy a home.

Under the proposals, London’s planners will consider releasing some stretches of the land, with conditions on affordable housing, energy efficiency, transport links and promoting access to green spaces.

Andrew Boff, from the London Conservatives said the idea was a “complete betrayal” of previous pledges to protect the green belt.”

Pressures on the housing market such as rising interest rates, landlord reforms and increasing demand have led in part to a rise in homelessness in the capital in recent years, as well as a hike in rents.

Sir Sadiq is expected to say that if done “in the right way”, building new homes on green belt land could “unlock hundreds of thousands of good-quality new homes for Londoners” and ease the housing crisis.

The announcement marks a change in stance for Sir Sadiq, who has previously argued against developing the green belt while there are still undeveloped brownfield sites available.

While the mayor is expected to continue prioritising brownfield land, he is expected to tell his audience: “We have to be honest with Londoners that this alone will not be enough to meet our needs.”

London has a new target of building 88,000 new homes a year, but the current plan, which relies on building on brownfield sites, is delivering only around 40,000 a year.

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.”

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, has backed the idea because she said there was a “desperate need to build new homes, particularly more affordable homes”.

Ben Twomey, chief executive of campaign group Generation Rent, said he believed the mayor was right to consider the idea, as housing costs were “forcing too many of us into poverty and homelessness”.

BBC London’s political editor Karl Mercer said the plans could put the mayor on a collision course with several outer London Conservative boroughs, like Bexley, Bromley, Hillingdon and Croydon – all four refused to take part in the mayor’s review of the green belt earlier this year.

Among the areas in London at risk of losing green belt under the plans is Enfield, where the local authority is planning a housing development of 3,700 homes in an attractive rural setting, Vicarage Farm.

Carol Fisk is campaigning against the plans and has accused Sir Sadiq of “going for growth at at all costs”.

She said: “We’re very disappointed that a mayor who has always been a staunch defender of the green belt throughout his mayoralty has made this u-turn.”

Speaking to BBC Radio London, Ms Fisk said: “Green belt is not the solution. There are many, many planning permissions outstanding, there are vast areas of brownfield land that haven’t been built on yet.”

Ms Fisk added that she believed rent control and further landlord reforms would be more effective in addressing the housing crisis.

Countryside charity CPRE, which opposes the plans said there were currently 300,000 planning permissions that had not been built yet.

Andrew Boff, from the City Hall Conservatives, added: “The mayor has repeatedly stood in the London Assembly chamber and given categorical assurances that he would protect London’s green belt.

“This announcement is a complete betrayal of those promises and shows utter contempt for Londoners who treasure these vital green spaces.”

David Simmonds, Conservative MP for Ruislip, Northwood and Pinner and shadow minister for Housing, Communities and Local Government said: “The mayor should be looking to bring forward those homes that already have planning permission in the capital rather than opening up the green belt, because once that is gone, it’s gone forever, and it’s a vitally important asset for present and future Londoners.”

Additional reporting by BBC London’s political editor Karl Mercer, Lauren Stanley and Susana Mendonça