The average landlord who sold their rental property in 2017 made a gain of £86,651, figures show.
Landlords in London gained significantly more at £253,981 – over four times more than those selling outside the capital.
In fact, one in four landlords who sold their home in London did so for at least twice what they paid for it an average of 8.1 years ago, according to the data from Countrywide.
Landlord gains are slightly behind owner occupiers, who on average made £92,886 when selling their home in 2017.
This is because the average landlord selling their property owned it for 8.7 years, rather than nine years for an owner occupier.
Eight of the 10 places where landlords made the highest percentage gains were in London, with Maldon in Essex (118%) and Pendle in Lancashire (109%) being the only exceptions.
Landlords in the North East made the smallest gains at £23,874, over 10 times less than a landlord in the capital.
The average cost of a new let rose to £954 per month in Great Britain in February 2018. But the rate of annual rental growth slowed to 1.5%, down from 2.4% the previous month.
Rents in London grew faster than any other region for the third month in a row to stand at £1,686, 3.1% above last year’s level.
Scotland was the only region to see rents fall in February.
Johnny Morris, research director at Countrywide, said in eight of the last nine years house prices have risen, which has driven investor gains.
“Even in areas where price growth has lagged behind most landlords have made a profit from rising prices,” Morris said.
“Rents continued to grow in January. London continues to see the greatest falls in the stock of available homes to rent, on the back of reduced investor activity, this scarcity of supply is driving rental growth.”