House prices in Chelsea have fallen by 9.8 per cent annually, according to Knight Frank.
Overall, prices for high-end homes in central London fell by 2.6 per cent in September, the estate agents said.
There were marked regional differences in prices for prime properties. In Hyde Park and Notting Hill prices fell by 7.5 per cent and 5.3 per cent respectively, but in Islington, prices went up by 3.6 per cent.
"Stamp duty remains a decidedly bigger influence on the market than the EU referendum and in some instances the uncertainty surrounding Brexit has been a catalyst for overdue price reductions," said Tom Bill, head of residential research at Knight Frank.
Properties are also taking longer to shift, with the average number of days a house stays on the market increasing 14 per cent between January and August this year as compared to the same period last year.
Bill said this was "a sign that some buyers remain cautious".
Rental values are also falling on high-end homes in London. In September, rental values for prime central London properties fell by 4.7 per cent year-on-year.
As with sales values, rents were hit particularly hard in Chelsea, falling by 8.3 per cent annually. In Marylebone, rents fell by as much as 9.9 per cent. In King's Cross and the City Fringe, rents were up by 1.9 per cent and one per cent respectively.