Zoopla sailed back into its relentless attack on OnTheMarket within hours yesterday of confirming to shareholders that by the end of March it had lost 23% of its UK estate agency membership.

In its latest onslaught, Zoopla cites its own analysis, claiming that listings and instructions have been climbing for agents who have stayed on both Zoopla and Rightmove portals.

By contrast, it says, the average number of listings per branch for agents with OnTheMarket has been falling.

Again citing its own research, Zoopla says that OTM agents’ market share of new instructions has fallen by 15%.

Zoopla also claims that agreed sales among OTM agents are down by up to 10%.

Ian Springett, chief executive of OTM, said: “Without more detail of this Zoopla ‘analysis’ it is hard to take it seriously and it seems simply to be another desperate set of unsubstantiated claims geared to stemming the flow of agents away from it as they join OTM.

“The claims are directly contrary to the consistent feedback that leaving Zoopla has made no difference to instruction-winning capability.

“The claims also seem to be at odds with announcements recently by Countrywide and LSL, both major shareholders in ZPG and with some 1,400 offices between them listed on Zoopla. As recently as April 30, it was reported that for first quarter 2015 compared with first quarter 2014, Countrywide experienced a 13% fall in exchanges and LSL saw income from sales fall 15%.

“Meanwhile, only this week Spicerhaart Group, a major group supporter of OTM, has announced a surge in new instructions.

“Moreover, analysis published recently by Home indicates that our member agents are selling the properties they are instructed on materially faster than those who remain non-members.”

Zoopla’s full press statement says: “New analysis from Zoopla Property Group (ZPG) shows that the average number of listings and new instructions per branch has been steadily climbing since the start of the year for agents who have remained with both Rightmove and ZPG, whilst the average number of listings per branch for agents who have joined OnTheMarket (OTM) has been steadily falling.

“Furthermore, separate analysis by ZPG into both the number of sales and share of new instructions of UK agents between September 2014 and March 2015, reveals that the market share of new instructions for OTM members has fallen by over 15% during this period whilst those that have remained with both ZPG and Rightmove have seen their level of new instructions stay at the same level.

“The data also highlights that whilst the overall number of agreed sales declined 4% during this period, agents listings on both Rightmove and ZPG have actually seen their levels of sold properties increase by 15% whilst some agents who have joined OTM have seen their number of sales agreed drop by as much as 10% during the period.

“Lawrence Hall of ZPG said: ‘The data shows that the majority of agents who have remained with the market-leading portals are in a far stronger position and are growing their market share as a result. This analysis provides compelling evidence of the impact of OTM’s ‘one-other portal rule’ on its members as many vendors and landlords clearly see the benefit of choosing an agent who provides the widest possible exposure for their home when entering the property market.

“It is no coincidence that we have already seen dozens of members return to ZPG having quit OTM due to the negative impact on their business and we welcome back any members who have realised that we are a key partner whose sole focus is on driving them more business.'”

Springett said of Zoopla’s half-year results, announced yesterday, he remained confident of OTM becoming the UK’s number two portal by the end of next January.

He also revealed that last week, OTM sent a record number of leads to agents. He said: “Every month since launch, our membership, traffic figures and property listings have increased.”

He added that in many areas across the UK, OTM has already overtaken Zoopla with its listings.

Yesterday, City analyst Canaccord Genuity cut Zoopla shares from a buy to hold rating.