Zoopla shares fell yesterday despite delivering strong results to its shareholders and giving them reassurance that it did not regard the launch of OnTheMarket on January 26 as more than a passing threat.

Although the shares initially went up 3%, the collapse set in as the day went on, with the shares plunging so much at one stage that estate agents who had bought at a 20% discount on the initial offer price of 220p saw their saving more than wiped out as the shares fell to 170p.

But the shares recovered to finish the day at 181.50p, down 11.40p (6%) on the day.

Rightmove shares – which had enjoyed a good week – also had a bad day, ending at 2,239.50p, a fall of 79p (3.4%).

There was some mystery as to the reason for the Zoopla fall and the extent of it after such confident results.

However, there was a conference between Zoopla with analysts mid-morning, and one later said there had been “some incremental negative takeaways from the call”.

The analyst, William Packer of Exane BNP Paribas, did not explain what these were.

At the conference, Chesterman told analysts that Zoopla has seen no discernible increase in churn ahead of the OnTheMarket launch.

Chesterman said: “What we do know is what it takes to be successful in this space.

“You need to have a whole-of-market inventory, which the leading portals have. You need to have a world-class platform with a very rich feature set, which we’ve invested tens of millions of pounds over multiple years in developing.

“And you need to have a huge consumer audience – in our case over 45 million visits per month, in order to deliver value to customers and make it worth their while advertising.

“We think that’s a challenging proposition for Agents’ Mutual along with any other entrant.”

Chesterman also told analysts that the new competitor could damage agents’ relationships with their customers, saying: “A proposition like Agents’ Mutual, which contains restrictive marketing rules and prevents an agent doing the best possible job that they can do for a consumer, is only going to serve to alienate consumers from those agents that sign up to Agents’ Mutual, and those agents are going to suffer as a result.”

But an unconvinced Packer said that despite yesterday’s positive results, the “bear case is broadly unchanged” with Zoopla being more exposed – as the number two portal player – to Agents’ Mutual.

The broker also noted that the “vast majority”of estate agency customers are on rolling one-month contracts with Zoopla.

His note advised that Exane has a “clear preference for Rightmove over Zoopla”.

It advised investors to “continue to avoid Zoopla heading into the Agents’ Mutual launch”.

A second broker, Citi, said Zoopla’s results were “solid” but said that as long as Agents’ Mutual remained an unknown quantity, Zoopla’s shares would struggle to perform.

Citi said that “time will tell”.

Agents’ Mutual issued its own response to Zoopla’s results – taking Rightmove into its sights as well.

Ian Springett, chief executive, said: “It is our belief that Rightmove and Zoopla have failed to acknowledge the strength of the dissatisfaction felt by many estate and letting agents over the existing portal market duopoly.

“This is evidenced by the fact that thousands of agents have already signed up to switch from them and use the new portal when it launches on January 26.

“From this date, neither Rightmove nor Zoopla will be able to claim they cover the entire market because many estate and letting agents will be leaving them to join OnTheMarket which will create a unique set of listings.

“In order for what is a new start-up to become established, our member agents commit to list with us and just one other competing portal.

“This is likely to lead to the existing two major portals losing significant advertising business as OnTheMarket grows over the coming years.

“OnTheMarket is a mutual organisation with a mission to provide a first-class service to member agents, to their clients and to the broad property-seeking public.

“We’ll be injecting much-needed competition to the marketplace and we’ll be providing consumers with a state-of-the-art search experience.”