Forbes, who has been Rightmove's chairman since 2005, sold 300,000 shares at an average price of £23.50. He retained 319,300 shares in the 14-year-old company, which has a market value of £2.3bn.
The share sale comes as Rightmove is facing increasing competition from its main rival Zoopla – which is planning to float on the stockmarket next month – and estate agents setting up their own property portal.
Zoopla, which is just six years old and includes the websites Zoopla.co.uk and Primelocation.com, last week confirmed its intention to press ahead with a £1bn London flotation in June.
The float could value Zoopla founder Alex Chesterman's stake at more than £80m. It will be Chesterman's second big windfall after the sale of Lovefilm, which he cofounded, to Amazon for £200m in 2011.
Daily Mail publisher the Daily Mail and General Trust (DMGT), which owns 52% of Zoopla, will also make a big return on its investment.
Rightmove is also under increasing pressure from estate agents increasingly objecting to the fees to advertise on the website. The six biggest agents in London – Savills, Knight Frank, Chesterton Humberts, Strutt and Parker, Douglas & Gordon and Glentree Estates – are preparing to launch their own website, Agents' Mutual, next January.
More than 1,800 estate agents' offices have signed up to Agents' Mutual and the new portal has raised more than £5m in seed capital from members. Agents already signed up account for 12% of properties on the market, and those representing a further 19% have expressed interest.
Agents' Mutual, which is run by PrimeLocation founder Ian Springett, will ban participating agents from advertising their properties on more than one of the other big websites, meaning agents must choose between Zoopla and Rightmove.
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