A list of more than 500 affordable homes that were planned but may now not be built, because of funding cuts, has been released.
The Homes and Communities Agency (HCA) released the list of homes affected by £450m of government cuts to housing announced on Monday.
London's Green Party said it would be "unfair" not to build the homes.
But Housing Minister Grant Shapps said: "We are in the business of offering genuine help the country can afford."
The HCA said it would review 522 new affordable homes that were planned "to determine which of those can be funded with available resources based on the aim of maximising affordable housing and achieving best value for money".
Of the 522 "affordable" homes planned across London, 186 were homes for low cost ownership, 142 were for social rent and 194 would have been new council homes.
A Department for Communities and Local Government spokesman said: "HCA regional offices will be assessing which other schemes can be progressed."
London Mayor Boris Johnson said that it would be "insane" to cut funding for affordable housing, at a Big Society event at City Hall on Monday.
The mayor's housing strategy aims to deliver 50,000 affordable homes before the 2012 Olympic Games.
The Green Party's Darren Johnson said: "The government has picked the mayor's pockets before he can deliver his promised 50,000 affordable homes."
"If the mayor believes cuts to housing are 'insane', it's time he argued for fairer taxation instead of swingeing cuts that could send our rents, mortgage costs and the housing benefit bill spiralling further upwards."
But the mayor's housing adviser said Mr Johnson would still be able to hit his affordable housing target.
Richard Blakeway said: "It's wrong to say this will impact on the mayor's target on delivering affordable housing.
"We are confident we will be able to achieve the target," he added.
"The mayor has in fact increased London's share of the UK funds for affordable housing."