(Sharecast News) - Property portal operator OnTheMarket announced on Thursday that, as at 30 October, it had listing agreements with UK estate and letting agents with more than 11,500 branches.The AIM-traded firm said that increase of more than 6,000 branches since its admission to AIM in February was achieved in less than nine months.It recently announced that Belvoir Lettings, the UK's largest property franchise, had agreed to advertise all of its UK residential sales and lettings properties at OnTheMarket.com, and would be actively promoting the portal brand with digital and branch-based marketing activity."Our first stage of growth has been focussed on rapidly expanding our agent base and we are pleased that the latest increase to 11,500 offices highlights the strength of agent support for an agent-backed portal," said OnTheMarket chief executive officer Ian Springett."With a scalable portal, strong funding and a progressive ramping-up of marketing activity, growth since IPO has accelerated and momentum is continuing."This gives us confidence as we look to the coming months."The company's board said new agents supporting OnTheMarket since admission had ranged from Belvoir, with more than 300 offices, to thousands of small, independent one-to-three office agent firms.Nearly 20 estate and letting agent firms with seven or more branches had signed up to list all their properties at OnTheMarket.com and promote the brand since the company's IPO.Those firms included Kings Group, Richard James, Anthony Martin Estate Agents, Merryweathers, James Pendleton, Howkins & Harrison, Robert Oulsnam & Company, Pattinson, Millerson, H&H Land & Property, Fine & Country Midlands, Townends, Leftmove Estate Agents, Andrews, The Property Centre, Beals and Bidmead Cook.OnTheMarket announced on 2 October that comparing September's performance with February's, ot had doubled contracted agent offices to more than 11,000, trebled monthly visit traffic to more than 17.4 million visits and quadrupled email and telephone leads to agents.The growth in OnTheMarket's agency branch base to date had been predominantly from offering free listings under short-term introductory trial offers, with a view to converting those to full tariff contracts when the value of its offering had been demonstrated.It said its sales team was continuing to see "encouragingly high levels" of agent support, and conversion to signed contracts when it presented the opportunity of listing on an agent-backed portal.