A fifth (19%) of landlords with three or more properties have prevented tenants from switching suppliers, which has led to them overpaying for energy.
New research also revealed more than one in 10 landlords admitted to denying private tenants their right to switch energy companies.
That has added at least £161 million to their gas and electricity bills, it claims.
More than a third (36%) of landlords believe naming a preferred supplier in rental agreements mean they can stop renters from switching.
While a preferred supplier clause can be used in rental agreements, tenants are under no obligation to use the named company.
The research from uSwitch also found 43% of landlords who prevent their tenants from switching say it is because the supplier is the choice of the landlord.
Around 5% – or 230,000 – of tenants who haven’t switched suppliers at their current address say it is because their agreement prohibits it.
It also revealed more than three quarters (79%) of renters didn’t receive any information on how to switch supplier at the start of their tenancy and 60% believe landlords should provide more information about managing utilities when moving in.
Dan Wilson Craw, Policy Manager at Generation Rent said: “When a tenant pays the bills, their energy supply is not the landlord’s to decide. Arrangements between landlords or letting agents and preferred suppliers are unlikely to result in a good deal for the person who ends up paying. Renters should research what savings they could make by using a different supplier and should not be afraid to switch.”
CEO at the National Landlords Association (NLA) Richard Lambert said they encourage tenants to change their energy suppliers if they find a better deal.
In a statement he added: “Our research – conducted with approximately 1,500 tenants – paints a very different picture of the issues than the one advanced by uSwitch. It shows that 77% of tenants have indeed switched utility provider before with or without their landlord’s permission, and that just 5% feel that either their landlord or a tenancy agreement clause has prevented them from doing so.”