London and the south west are the only parts of the UK where rents are falling.
In February, the average rent in the capital was £1,280 per month, one per cent lower than it was at the same time the year before. Rents fell 0.5 per cent from January's figure, according to the figures from Your Move.
Prices were down 1.5 per cent in the south west, hitting £662 per month.
The decline shows rents have hit the "affordability ceiling" in London, Your Move said, while rents continue to climb in the counties. In the east of England, for example, rents have jumped 5.6 per cent over the past year, to an average of £878 per month.
Rents are rising fastest in Wales, and were up 7.7 per cent year-on-year in February, averaging £593 per month. The only places cheaper than Wales were the north east (average of £545 per month) and Yorkshire and Humber (average of £566 per month).
Valerie Bannister, lettings director at Your Move, said: "The dramatic rent increases in London have now slowed as people look outside the capital in order to meet their housing aspirations.
"Areas in the south east and east of England have traditionally offered much better value than the capital and this has tempted many Londoners to look further afield for rental properties. Renters in London could be reaching the limits of their affordability...This will be one to watch as the year progresses."
The trends broadly mirror house prices across England and Wales, with values in the centre of London dropping. However, the figures for average rents and house prices mask the changes happening within London itself. House prices are still rocketing in the outer boroughs such as Bexley and Barking and Dagenham, but have fallen by more than 10 per cent in areas such as Chelsea.