Zoopla could approach sellers and landlords and allow them to advertise directly on the site for a “fair, but high” price, a commentator has argued.

Simon Baker, who is behind the global online publication Property Portal Watch, said that this strategy could be deployed by Zoopla if it loses listings to OnTheMarket.

He said on his site yesterday that consumers could be offered the chance to advertise directly on Zoopla or go to their agent to get it cheaper.

Baker said: “Clearly this would put pressure on the agents to justify to their customers why they are not advertising on Zoopla – especially when the cost per listing is so small and the commission for the sale is relatively large.”

He said this approach would have to be backed up by a marketing campaign along the lines of: Is your listing on Zoopla? Only £x per month or ask your agent for a better deal.

Baker goes on: “They could even target the vendors of those agents who have not renewed directly through a good old fashioned direct mail campaign as the addresses are known!”

He said that Zoopla is run by a smart operator in Alex Chesterman who will not lie down “and let someone else eat their lunch”.

He also points out that Zoopla has significant funds of £31m in cash plus strong monthly cashflow.

Baker said: “If they are losing agents, then clearly it has to do something to get the listings back.”

One strategy, he says, would be for Zoopla to discount. But long term, this would simply dampen revenue streams and impact on the share price.

Going to vendors and landlords direct would, he argues, be a more preferable approach – and a means of getting listings up and agents back, should that be needed.

Baker concludes that there is a lot to lose by doing nothing and letting OnTheMarket through the door. “Time will tell – however, don’t ever underestimate a portal under attack.”

His blog is here