The upward pressure on private rents is predicted to cool down this year.
It will be helped by large numbers of "accidental" landlords who have developed a taste for renting out their homes, a property website says.
One in eight (12%) of "accidental" landlords, many of whom originally took on tenants because of difficulties selling their house in the tough economy, said that they now plan to buy another property specifically as an investment this year, Rightmove found.
Three-quarters (74%) of professional landlords also said they intend to expand their portfolios in 2013, giving a further boost to the available rental housing stock.
Miles Shipside, director of Rightmove, said: "Overall supply of greater rental property coming on tap is good news for tenants as it is likely to lessen the pace of rental growth in 2013."
The website estimates that about three in 10 landlords are "accidental". Encouraged by good rental returns, some 68% of them said that they intend to hold on to their current rental property and have no particular plans to sell.
Mr Shipside said: "Many of those accidentally thrust into the landlord arena have not only survived but thrived in many instances. The growth in rental demand and appeal of the buy-to-let sector to lenders has helped turn a chance encounter with the rental market into a further and valuable source of fresh supply."
A lack of choice for private tenants has been one of the driving forces behind strong rent increases in recent months.
People have remained in the rental sector for longer as many would-be first-time buyers have struggled to raise the typical 20% deposit needed to buy a home. This has created more demand in the sector, which has pushed rents up higher, making it even harder for tenants to save for a mortgage deposit.
Research from housing charity Shelter last week found that annual private rents have risen by almost £300 year-on-year on average across England and in some areas they have increased by thousands of pounds. Shelter has called for more Government action to release families from the "rent trap" which is holding them back from owning their own homes.