Rightmove said prices for flats and houses advertised between 6 December and 9 January were down 0.9% on the previous month.
The property website said that dragged its measure of annual growth to 3.3%, from 6.6% a month earlier.
Rightmove reported that the market was still busy, with the number of buyers contacting agents up 12% and sales agreed in January so far up 9% on the same period last year – when the market had also been strong after the December 2019 general election result.
Transactions slumped earlier in 2020 as the first coronavirus lockdown took its toll but the reopening of the market and a stamp duty holiday announced in the summer by chancellor Rishi Sunak helped revive demand.
The policy saw the threshold for paying stamp duty temporarily raised from £125,000 to £500,000 until 31 March this year.
Rightmove said the fall in asking prices came “with some new sellers still hoping to tempt buyers and squeeze in a sale before the stamp duty holiday ends”.
However it said that new buyers looked unlikely to beat the deadline with the average time to complete a purchase now standing at over four months.
Rightmove predicts that with more than 600,000 properties sold subject to contract, around 100,000 will face an unexpected tax bill as they miss the 31 March cut-off date.
Tim Bannister, Rightmove’s director of property data, said: “As we enter the new year and a new lockdown, the housing market remains open but is focused on the imminent end of the stamp duty holiday and on the challenges of the pandemic.
“These major influences on mover behaviour are clouding the 2021 outlook, but Rightmove’s early January market-leading indicators of buyer demand and the number of actual sales being agreed are looking robust.”