The average rent of a new letting in the UK grew by 0.24 per cent in the first six months of 2017, just a third of the growth seen during the same period in 2016.
The claim, by Landbay, suggests that the slowdown began as long ago as April 2015 when the first of the recent spate of tax changes for the private rental sector was announced by then-Chancellor George Osborne.
London has led the rest of the UK in falling rents, with June marking one full year since month on month rental changes in the capital first entered negative territory.
It remains the only region in Britain to have seen rents actually fall, by 0.59 per cent in the first half of 2017 and by 1.01 per cent over the full past 12 month period.
This compares to growth of 0.69 per cent growth across the rest of the UK in H1 2017 and 1.59 per cent over the past year.
Nine London boroughs are still seeing rents rise, with Havering (0.73 per cent), Waltham Forest (0.61 per cent), Bexley (0.58 per cent), Barking and Dagenham (0.41 per cent) and Enfield (0.20 per cent) experiencing the greatest uplift over the first half of 2017.
Outside of the capital, rents have yet to begin falling, but there has been a notable deceleration in growth.
England (without London) saw rents increase by 0.70 per cent in the first half of 2017 compared to 1.14 per cent in H1 2016.
In Wales, growth slowed to 0.59 per cent compared to 0.83 per cent the year before, while in Scotland rental growth has remained fairly steady, slowing by only 0.04 per cent from 0.69 per cent in H1 2016 to 0.65 per cent in H1 2017.