A number of agencies that joined OnTheMarket but subsequently stopped listing properties on the portal have received letters from a legal firm acting for Agents Mutual.
The letters, tailored to different clients, say that the firms are not listing properties in accordance with the terms of listing agreements reached with Agents Mutual, the parent company of OnTheMarket.
The letters also demand payment for membership fees that are alleged to be in arrears: one agent who contacted Estate Agent Today says he is expected to pay around £7,000.
The letters - dated this week - ask for payments of the alleged arrears within the next 14 days and say legal proceedings will commence to recover the sums if payment is not forthcoming in that period. The solicitors warn that the cost of the action to recover the sums will increase the alleged debt owed.
The agents who have received the letters - at least five known to Estate Agent Today but possibly many more - are told that they are also required to pay future membership fees as they fall due throughout the rest of the contract, and that any additional membership obligations set out in their contracts must be adhered to.
OnTheMarket has declined to say how many agents it is now pursuing and the volume of money it intends to extract from them in total.
But a spokesperson told us: "It is standard practice for any company to seek outstanding payments from customers when they are in arrears. We work to ensure that everyone in contract with us meets their obligations for the benefit of all members of Agents' Mutual."
Recent traffic figures reported by Rightmove and Zoopla have shown both heavily consolidating their dominance.
Rightmove reported last week that visits to its site in the early part of the year, up to early March, was up 14 per cent on the same period of 2015. There were a record 1.3 billion visits to the portal in 2015, a rise of 18 per cent or 200m over 2014. A record 49.8m leads were generated for advertisers - mostly agents - compared with 42.8m the year before, representing a rise of 16 per cent.
Zoopla says it recorded a 61 per cent leap in appraisal leads in February compared to the same month last year, and generated over 30,000 instruction opportunities last month alone. General traffic to its Zoopla and PrimeLocation sites was up 9.0 per cent compared to February 2015, with over 50m visits during the month.