OnTheMarket says it now has the support of over 7,000 estate and letting agency offices.
This includes those formally contracted now and those who have signed letters of intent to join Agents' Mutual when the figure hits 7,500.
A statement from the portal says its office tally has grown by over 1,650 since its launch in January 2015 and that in April it enjoyed its highest-ever number of visits - 7.25m according to Google Analytics.
OTM chief executive Ian Springett has renewed his attack on what he calls "detractors" saying that: "It might be disappointing for some of our detractors but OnTheMarket is here to stay and, in spite of fierce competition, is growing steadily both in terms of consumer visits and agent support."
He says that the sooner agents join OTM "the more powerful the portal will become in continuing to disrupt the duopoly and in providing a first-class alternative search service for consumers and agents alike."
Meanwhile the convenor of an action group of disgruntled OTM member agents, Iain White, is meeting with legal advisers and over 60 agencies this morning in central London.
White says that in addition there are around 25 agencies interested in a possible class action around alleged disputed fees and performance issues concerning OTM, but unable to attend this morning's meeting.
He says there are around another 100 or so agents who have made contact with the action group and are currently involved in individual discussions or action with the challenger portal, and are not at this stage involved in any collective approach.