The number of landlords with smaller portfolios has decreased, following tax changes in the buy-to-let sector.
Research by Paragon found that the proportion of landlords with 3 to 5 properties down from 26% to 24% at the end of 2017.
This contrasted with rising figures for those with slightly larger portfolios, as landlords with 6 to 20 properties increased from 35% to 39% in Q1 2018.
According to John Heron, managing director of mortgages at Paragon, this "polarisation" in the sector has been largely driven by tax and regulatory changes.
In a separate survey by Aldermore, 25% of landlords said changes to tax relief were their main challenge, while 22% cited increased stamp duty.
Others were mainly concerned with changing pressures in the sector, such as growing pressure on yield (15%), pressure on rent (14%), and increased competition (11%).
Charles McDowell, commercial director of mortgages at Aldermore, said:
"There is no denying the buy-to-let market has taken a bit of a battering, thanks to a multitude of regulatory, underwriting and tax changes.
"Despite the recent changes, many still view buy-to-let as a good investment, with expansion on the horizon, particularly among those who are specialists in this area."
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