Homeowners planning to sell in the new year may find themselves the victims of a property website battle sparked by the arrival of an alternative to Rightmove and Zoopla.

On 26 January, a consortium of big-name estate agencies will launch a property listings website, Onthemarket.com, in an attempt to break what they describe as the “duopoly” of the two big property portals.

However, agents that sign up with the site will be forced to choose between its two rivals, which means sellers could see their homes delisted from one of the sites, which between them get 100m hits a month. As a consequence, consumer group HomeOwners Alliance, has described it as a “rotten deal for homeowners that lines estate agents pockets”.

“When selling your home from next year you won’t be able to have access to all the portals with certain estate agents,” said Paula Higgins, chief executive of HomeOwners Alliance. “That restricts the number of potential buyers homeowners can market their house to. This in turn will impact the speed of sale and getting the right price.”

Agents’ Mutual, a not-for-profit organisation run by Savills, Knight Frank, Chestertons, Strutt & Parker and the London firms Douglas & Gordon and Glentree Estates, says more than 4,000 agents have signed up to the website and more are being recruited.

The website is offering members the option to fix the monthly fee for listing properties for five years in what it describes as a chance for agents to control costs in the face of escalating Rightmove and Zoopla fees. However, as a condition of membership, agencies will be forced to sever relationships with either Rightmove or Zoopla, whose parent group also owns Primelocation.com, on or before the website launch date of 26 January.

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