Zoopla, the British property website, said it has been affected by the launch of rival OnTheMarket, with the number of agents advertising on its website falling 11 per cent.
OnTheMarket was set up and launched last month by Agents' Mutual - a not-for-profit organisation founded by agents including Savills and Knight Frank - to challenge the dominance of Zoopla and Rightmove.
OnTheMarket promised lower fees, but only lets members remain on one of the "big two" sites and excludes online-only agents.
Zoopla, the UK's second largest property website after Rightmove, said it had been "impacted in the short-term" by OnTheMarket and its "restrictive 'only one-other portal' rule".
It said total advertising members at the end of January had fallen to 16,967 from 18,999 at the same time last year.
The company, which listed last year, said there could be some further UK agency churn, but that could be partly offset by members finding OnTheMarket's membership policy too limiting.
Zoopla's chief executive and former LoveFilm entrepreneur AlexChesterman tried to play down the effect of OnTheMarket's launch, calling it a "short-term event".
He said: "We believe that Agents' Mutual remains a short-term event and that its success will be determined by whether consumers engage with it. So far it appears to have got off to a very slow start with consumers."
The company otherwise produced a strong trading update, reporting record levels of consumer traffic with 50.5 million visits in January.
It also said its users are becoming more mobile, with 60% of customers using mobile devices to access the site, up 42 per cent year-on-year.
The figure is at odds with data showing a slow down in the property market as a result of lenders tightening up loan criteria and speculation over an interest rate rise.Reuse content