Estate agents have accused Rightmove of charging "unsustainable" fees, with some saying their charges have more than doubled in recent years.
The online listing portal is now being pursued in a class action, launched by accountant Jeremy Newman on behalf of potentially hundreds of estate agents.
The BBC understands a letter of claim has been sent to Rightmove seeking just under £1.5bn in damages, claiming the website has "abused a dominant position" in the online property portal market.
Rightmove said the claim was "without merit, and we will defend it vigorously", adding it is "confident in the value we provide to our partners and consumers."
The claim alleges that Rightmove has charged thousands of estate agents and new home developers excessive and unfair subscription fees, and action has been filed in the Competition Appeal Tribunal.
Newman claims hundreds of estate agents are reporting a rise in fees with little change in the services they are receiving while they have been "squeezed" over the past few years by flat property prices.
"Estate agents are having to employ fewer people because they can't afford them alongside their fees to Rightmove," said Newman, who is also a former Competition and Markets Authority (CMA) panel member. "As a result, their services can't be as effective."
Legal action against Rightmove.
Rightmove is now facing legal action led by Jeremy Newman, a former panel member at the Competition and Markets Authority.
Jeremy is representing national and local estate agents alike in a case that is fully funded by Innsworth Capital Limited and supported by leading lawyers and experts, including Scott+Scott UK LLP.
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