The estimated number of landlords in the private rental sector has shrunk to a seven-year low of 2.66 million, according to a report by Hamptons International.
Investors in Scotland were among the least likely to have big buy-to-let portfolios, with landlords there and in Wales letting out 1.83 properties on average.
Hamptons said the overall figure was the lowest level since 2012 when there were 2.58 million landlords in Britain.
The number of landlords peaked at 2.88 million in 2017, but the report said tax and regulatory changes have caused some landlords to sell up and leave the sector.
Despite the falling number of landlords, those remaining in the sector are building bigger property portfolios.
Three in 10 (30%) landlords owned more than one buy-to-let property in 2019 - double the proportion seen a decade ago when just 15% owned multiple rental homes.
The average landlord owned 1.93 buy-to-let properties last year, the highest number since 2009.
Landlords in the North East of England were found to have the biggest portfolios, at 2.05 homes on average, followed by those based in Yorkshire and the Humber (2.03) and London (2.01).
Researchers also found that average rents across Britain increased to £998 per month in January, up by 3.6% annually.
Rents increased across Britain, from a 6% annual uplift in the South West of England to a 1.2% upswing in Wales.
Aneisha Beveridge, head of research at Hamptons International, said landlords "who have stayed tend to have bigger portfolios - a further sign that the sector is professionalising".
She continued: "The number of new homes purchased by landlords remains low, which is feeding through to fewer homes available to rent. This is particularly true in the South, where rents are rising the most."
