The GMB said its research in Barnet shows how the right to buy has turned into a "rich harvest for the greedy elite".
The situation will be repeated under legislation going through Parliament extending the right to buy to Housing Associations, warned the union.
Its study of 768 houses in Barnet showed that 224 were now owned by private landlords, who rent them to tenants.
One landlord owns nine houses, while 12 own more than one, the investigation of Land Registry records found.
Gary Doolan, the GMB's political officer, said: "This investigation lays bare the harsh reality of the exploitation of our social housing stock.
"It shows private businesses making vast profits from the public purse while the people these homes were built for sit on waiting lists that never move.
"To add insult to injury, many are using offshore tax havens to avoid paying tax on these profits.
"These homes were built and designed to be rented by ordinary London families. The investigation exposes Margaret Thatcher's flagship policy of right to buy for individuals as nothing more than a charter for the exploitation of our social housing for private profit.
"Her plan to create a home owning democracy has turned into a buy to let bonanza.
"While tens of thousands of ordinary London families are unable to find council house to rent and the elite are scooping them up by the tractor load.
"The right to buy has turned into a rich harvest for greedy elite and the same will happen with the properties acquired from Housing Associations under legislation currently in the House of Lords."