As a result, the average size of a landlord portfolio has grown to a new recent high of 1.44 properties, according to Countrywide.
Its Monthly Letting Index for August shows the number of landlords has fallen over the last two years despite a rise in supply of homes available to rent.
Countrywide estimates that the number of landlords peaked at 3.72 million in 2015 when there were some 171,000 fewer rented homes than today.
In 2017 there are just over 154,000 fewer landlords (3.56 million in total) but the number of rented homes has increased from 4.9 million in 2015 to 5.1 million. (See graph below, click to expand.)
The number of landlords owning just one buy-to-let property has shrunk to 73% (down from 86% in 2010), while the number who own 10 or more has risen by a third (33%) in the last decade.
Average rents for new lets rose to £954 pcm in August, up 1.6% on the same time last year, with those in the South West (4.7%), Scotland (2.8%) and East of England (2.5%) growing sharpest.
London rents also grew for the second consecutive month, up 1.8% year-on-year.
Countrywide research director Johnny Morris (pictured) said: “Increasing regulation in the sector accompanied by recent changes to income tax relief on mortgage interest payments seem to be favouring more experienced, professional landlords.
“Despite expanding portfolio sizes the sector is still characterised by those owning just one or two homes, 73% of landlords own one home.”
However, he noted the number of landlord purchases continued to remain low which was feeding through into fewer homes on the rental market.
“Rents in London rose for the second consecutive month, driven by a pickup in rents in outer London,” he added.