Source: Sharecast
Rightmove said it was aware of reports of an application to begin collective proceedings against it.
In a brief statement, it said: "Rightmove is confident in the value we provide to our partners and consumers, who are at the core of our business solutions and digital platform.
"As one of the most efficient parts of the UK housing market, we help people across the UK to move home by bringing buyers, sellers, renters, landlords and agents together.
"Our platform continues to provide a growing range of constantly evolving products and features which facilitate market transparency, liquidity and confidence. This claim is without merit, and we will defend it vigorously."
Rightmove did not disclose details of the legal action, but according to the BBC, estate agents have accused the property portal 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 reported.
It was understood that 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.
The claim alleges that Rightmove has charged thousands of estate agents and new home developers excessive and unfair subscription fees.
According to the Solicitors Journal, specialist litigation funder Innsworth Capital Limited has filed the claim against Rightmove.
At 1025 BST, the shares were down 7% at 399.07p.
See latest RNS on Investegate