Landlords are still collectively costing tenants millions in unnecessary energy costs by preventing them from switching supplier, a survey suggests.
More than one in ten landlords (13%) admit denying private tenants their right to switch energy supplier, adding at least £161 million to renters' bills, uSwitch has calculated.
The figure rises to a fifth of landlords (19%) who rent out three or more properties.
Landlords cited a high turnover of tenants and even late rental payments as reasons for denying a change of supplier, while more than a third (36%) incorrectly believed naming a "preferred supplier" in the tenancy contract meant they could stop renters from switching.
Some 5% of renters - or an estimated 230,000 - who have not switched said it was because their tenancy agreement prevented them from doing so.
Ofgem regulations state that tenants who are responsible for paying their energy bills are allowed to switch supplier, and landlords and letting agents cannot unreasonably prevent them from doing so.
A "preferred supplier" clause can be used in rental agreements, but tenants are under no obligation to use the named provider.
USwitch said its survey revealed confusion among landlords about tenants' rights, with 43% of those who prevented their tenants from switching claiming the supplier was their choice, and 40% wrongly stating it was because they included a "preferred supplier" clause in their tenancy agreements.
It is calling for an "urgent" review of misleading terms in tenancy agreements, better training for landlords and more information for tenants about their rights.
Ann Robinson, director of consumer policy at uSwitch.com, said: "Landlords who unfairly refuse tenants their right to switch are standing in the way of more affordable energy for millions of homes.
"With a £339 difference between the average standard tariff and the cheapest deal, there has never been a more important time to help the growing population of renters tackle the sky-high cost of energy.
"Given that tenants are half as likely to switch as homeowners, any measures to break down the barriers and encourage them to take more control of their energy will reduce bills by millions."
:: Opinium surveyed 1,000 private tenants and 500 private landlords between September 2-7.
A Government spokesman said: "There's no reason why landlords should deny tenants choosing a supplier for a service they themselves are not using. Consumers have a choice and families are wise to shop around for the best deals.
"When entering into a tenancy agreement tenants should check who is responsible for paying electricity and gas bills and notify meter readings to the utility company."