Council houses have been sold off much too cheaply under Right to Buy, at valuations seven years out of date.

There have been discounts of up to 70% on more than 130,000 former council homes sold off in the last decade.

Altogether the price discount has been nearly £4bn – more than Margaret Thatcher could ever have envisaged, the Guardian reports.

Between 2012 and 2014, the London borough of Tower Hamlets sold off over 50 council homes using out-of-date valuations, according to data released under Freedom of Information Act.

The sales included a two-bedroom flat in 2012 based on a seven-year-old valuation – a period in which local property prices rose 30%.

Tower Hamlets blamed the use of out-of-date valuations on resourcing issues, but said the time lag has now been reduced.

Tom Copley, Labour housing spokesperson at the London Assembly, estimated that half of all properties sold under Right to Buy in Tower Hamlets are now being let out.

Another London borough, Sutton, revealed in a Freedom of Information response that it had sold off several flats at a 70% discount.

According to Inside Housing magazine, 38% of council homes sold off under Right to Buy across 91 local councils are now being let out privately.

The Guardian’s report comes as David Cameron aims to extend Right to Buy to social housing tenants.