Knight Frank, one of the founding agencies behind Agents’ Mutual, is to drop Zoopla when OnTheMarket launches on January 26.

Under the “one other portal” rule, most agents are effectively having to choose between the existing big two.

Knight Frank’s announcement follows a similar announcement by Spicerhaart, and was welcomed by Rightmove.

A spokesperson for Rightmove said: “We are pleased to confirm that Knight Frank will be advertising on Rightmove well into 2016.

“We look forward to continuing to work closely with Knight Frank and assisting in the promotion of its marketing objectives.”

Noel Flint, partner at Knight Frank, said: “Knight Frank has decided to continue utilising Rightmove in addition to OnTheMarket, the new property portal, which launches at the end of January.

“After consideration of the data available to us, we noted that Rightmove generates considerably more referrals to our website than its sole rival in the existing market, Zoopla.

“Feedback from our agents across the country also confirmed that Rightmove was the best choice available to us at this time and, most importantly, the best decision for our clients.

“The launch of OnTheMarket.com in the New Year is a significant step for the property market as the creation of a new portal will offer consumers more choice and challenge the existing duopoly.

“It is our belief that a property portal operated on a not-for-profit basis and existing purely to promote the properties from the best estate agencies can only benefit sellers and landlords while also offering the best entry point to the market.”

City analyst William Packer, of Exane BNP Paribas, said this morning: “A key London agent leaving Zoopla suggests the PrimeLocation dynamic may be less important than some Zoopla bulls have argued.”