Housing developments that segregate poorer tenants from their wealthy neighbours through the use of “poor doors” are still being given planning permission despite both the mayor of London and the prime minister suggesting they should be banned.

Developments with separate blocks, entrances and amenities for residents of social housing have been given the green light by almost all of the London councils that responded to the Observer’s request for information on their planning policies. This is despite growing calls for these homes to be indistinguishable from those being sold on the open market.

Earlier this year, Theresa May, while pledging more money towards social housing, said it should be impossible to tell the difference between full-price and affordable housing. The latter should not be “tucked away, out of sight and out of mind”. Her words echoed those of London mayor Sadiq Khan, who included a commitment in his 2016 mayoral manifesto to “support ‘tenure-blind’ development, avoiding the use of ‘poor doors’ ”.

London mayor Sadiq Khan vowed to support ‘tenure-blind’ housing. Photograph: Victoria Jones/PA

However, research by the Observer shows that many London councils are choosing to ignore this, instead allowing property firms to segregate families eligible for affordable housing from private residents with separate entrances, blocks and outdoor spaces. Eighteen of the 19 boroughs that responded to the survey said their planning polices did not prevent separate entrances for different tenures on mixed developments. Only Enfield claimed to have guidelines in place to block poor doors, while Croydon said it would not support separate entrances unless a housing association requested them. Others, including Islington, said they would promote tenure blindness – where all homes look the same regardless of whether they are social or market-rate housing – but some schemes might justify a separate entrance or communal space.

Examples of developments under construction with poor doors include:

• Luxury flats and houses on the site of a former workhouse in Fitzovia, central London. Private owners will have their own courtyard and gated front entrance on to the main street while less well-off tenants in 40 mostly social flats will use a side entrance at the end of a public alley.

• The Altitude scheme in Hornsey, north London, which is made up of three separate blocks of private, shared ownership and affordable flats. The main entrance to the affordable block is on a different side of the development to the other entrances, with the most direct route to the front door down a side path, which was included so bin collectors could access the building’s refuse store. The development was signed off in the face of criticism from one of Haringey council’s own housing officers.

• Smithfield Square, also in Hornsey, includes 2,290 square metres of “private landscaped courtyard gardens” for private flat owners, which is nearly three times the size of the 890 square-metre courtyard for housing association tenants living in separate blocks on the same site. The scheme was approved by Haringey despite the housebuilder Berkeley estimating there will be 123 children using the smaller courtyard compared with only 18 children from the private flats.

Residents of one of the schemes spoke of their dismay at the segregation. Millicent, who did not want to give her last name, moved into her housing association flat with her children last year and said she had not met anyone from the private block: “They tend to keep themselves to themselves. It shouldn’t be like that.No one should be treated differently, we’re all human.”

Venelina Liu, who rents in the private block, said: “I haven’t met anyone from the other blocks. It’d be much better if people lived side by side.”

Many of the councils surveyed claimed the only way to avoid expensive service charges was to allow developers to include separate blocks and separate entrances for social and affordable tenants.

Paul Burnham, secretary of Haringey Defend Council Housing, said councils should stand up to developers who want to segregate the better-off from the less well-off. “Tenants’ stairwells facing away from the centre of the estate, and segregated ‘bins entrances’ like at the Altitude development are just obnoxious and unacceptable,” he said.

He added that service charges were being used as an excuse for segregation. “Council planners need to ensure that service charges are ‘capitalised’ or integrated into the overall development costs,” he said. “If they do that, then people can live side by side.”

Haringey council said it had found that housing associations would not take on the management of “pepper-potted” homes [affordable housing spread around a mixed-tenure development].

However, it should “not be possible for affordable housing blocks to be distinguished from private blocks when viewed externally”. It added that the number of children expected at the Smithfield Square development had been taken into account when it made its planning decision.