Concerns have been raised over "deposit-free" renting being mis-sold to tenants who do not understand the costs involved.
The option allows renters to pay a smaller, generally non-refundable fee in place of a traditional deposit.
Companies and letting agents offering it say it reduces upfront costs.
But campaign groups and the ombudsman have warned some are pushing deposit-free options without making it clear they could cost renters more over time.
A traditional deposit is typically up to five weeks' rent - but this is refunded at the end of the tenancy, minus any deductions for damage or unpaid rent.
