Lettings in the high-end of London's property market have been "subdued" in the second quarter of the year due to both a fall in demand and an increase in supply.
JLL said "immaculate properties presented in first class condition" are not losing value, but on average rental values fell by 1.9 per cent during the second quarter.
In the 12 months to the second quarter, values dropped by 4.3 per cent.
The number of transactions in the prime central London lettings market fell by one per cent in the 12 months to the first quarter as compared to the same period last year.
Neil Chegwidden, residential research director at JLL, said: "The main feature of the current market is an oversupply of stock. With weakened tenant demand, the increased supply of properties on the market is not being eroded.
"Available supply has also been boosted by owners electing to rent out their properties as opposed to selling them, given the diminished demand in the sales market."
JLL said the outlook for the third quarter was far more positive as wealthy students and families moving into the capital for the start of the school year snap-up high-end properties.
Lucy Morton, director of residential agency at JLL said there was a "marked increase" in enquiries from relocation agents acting for City businesses moving expats into London.