The purchase by the Universities Superannuation Scheme, which invests the retirement pots of half a million lecturers and researchers, is the largest affordable housing deal of its kind since the shared ownership scheme was created in 1990.

The sale comes after Blackstone agreed to purchase 1,750 rental homes from housebuilder Vistry last month.

Blackstone said the portfolio was sold by its Sage Homes vehicle – a housing joint venture with investor Regis that was launched in 2017.

Sage has spent nearly £4 billion to own 17,000 homes nationwide, about 11,000 of which are rented out at affordable or “social” rents. The remainder are under shared ownership, a scheme designed to help people to get on to the housing ladder by allowing buyers who cannot afford an entire property to buy a share of their home with the option of increasing their ownership over time. They pay rent on the portion of the home they do not own.

James Seppala, Blackstone’s head of European real estate, said the money raised from the sale to USS would be reinvested in acquiring more affordable homes.

“Through Sage Homes, which was established in 2017, Blackstone has been the largest provider of newly built affordable housing in the country for the last three years,” he said.

“By deploying capital to fund the development of new homes, we are proud to have created an institutional-grade portfolio, which, in turn, has attracted more long-term institutional capital into the sector. This transaction will allow us to continue to invest capital into Sage Homes to help to alleviate the structural undersupply of housing across the UK.”

The 3,000 or so flats and houses are the “seed” properties for Sparrow Shared Ownership, the new social housing company launched by the USS this week. Sparrow will be chaired by David Avery, who has sat on the boards of several housing associations over the past two decades.

Eamon Ray, the head of private credit and alternative income at USS Investment Management, said that Sparrow would “provide further capital into the UK shared ownership sector”.