Croydon could miss out on 145 new affordable homes as part of recently approved plans by Hammerson and Westfield to redevelop the Whitgift Centre.
Under the new £1.4bn project, up to 967 new homes will be built – with currently only 20% will be affordable.
It falls short of Sadiq Khan’s pledge for 35% for developments on public land, though he accepts it is a “starting point.”
A charity has urged him to “claw back” as much affordable housing as possible.
Mr Khan expects the amount of affordable housing in the Croydon project “to rise over time”, something which Brian Robson, of the Joseph Rowntree Foundation, backs.
Mr Robson said: “I’m sure there are reasons as to why the deal was struck this way, but the mayor has been really strong in getting developers to stick to these affordable housing targets since coming into office.”
He said he used his “full range of planning powers” to bring the level of lower-priced housing to his required minimum of 35%.
Since becoming mayor, Mr Khan has intervened with three other developments – in Wealdstone, Tottenham Hale and Mill Hill – in order to raise the amount of affordable housing in.
Although the Westfield project was referred to the mayor, rather than “called in”, Mr Robson has urged Mr Khan to take a similar approach with the Croydon development – which will see the existing Whitgift Centre, on private land, be demolished.
City Hall said, in terms of the Westfield project, the mayor “accepted a starting point of 20% affordable housing to get this major regeneration scheme for Croydon under way as soon as possible”.