The High Court found they’re liable for overcharging tenants on their water bills.
The authority receives a commission from Thames Water for handling the services, in an agreement that sees the charges reduced.
However, it’s been found they hadn’t been passing these savings onto residents.
Kingston says they “accept the findings” and will be publishing information on how people can claim a refund.
It means people in other boroughs can claim refunds too, with Southwark reportedly repaying 41,000 tenants nearly £29m.