Landlords seeking to rent flats and other smaller properties as house shares are facing new rules.
More than one million people across England have suffered at the hands of a rogue landlord in the last year, a housing charity estimates.
The private rental market continued to show signs of strength as rents in London alone increased 3.8% over the last year.
Calls have been made for rental sector tenants to be given more legal protection alongside powers to force rogue lettings agents out of business in the worst cases.
Which? said that lettings agents should be governed by the same laws as estate agents and the Office of Fair Trading (OFT) should have the power to ban lettings agents who break the rules.
There are 4.7 million UK households in the private rental sector and two thirds of private tenancies involve an agent.