Landlord numbers have been shrinking but those who are still operating are expanding the number of properties they own, research suggests.
Lettings network Countrywide said the number of landlords has fallen over the last two years despite a rise in supply of homes available to rent.
It said fewer landlords combined with more rental properties means the size of the average landlord's portfolio is the biggest since its records started in 2005.
The average landlord owned 1.44 rented homes in 2017, up from 1.33 in 2015 and a low of 1.24 in 2010, Countrywide said.
It estimates that the number of landlords across Britain peaked at 3.72 million in 2015, compared with 3.56 million in 2017.
But it also calculates that the number of rented homes has increased, from 4.9 million in 2015 to 5.1 million now.
In 2015, 80% of landlords only owned one property, but by 2017 that proportion had fallen to 73%, according to Countrywide's estimates.
Landlords in the North East of England, Yorkshire and the Humber and London are particularly likely to have larger portfolios, the research found. Landlords in London are particularly likely to have more than 10 homes, Countrywide said.
Recent changes to stamp duty and other taxes have pushed up costs for landlords.
Some "accidental" landlords who do not rent out properties as their main job may have also decided to sell up in recent years as house prices have increased.
Johnny Morris, research director at Countrywide, said: "The increasing number of rented homes is being driven by landlords expanding their portfolios rather than new landlords entering the market.
"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."
The index is based on the 90,000 homes let and managed by Countrywide each year.