The pair sold 100 million shares for £366m according to Reuters, which cited Deutsche Bank, one of the banks handling the sale alongside Morgan Stanley.
Merlin runs attractions including Madame Tussauds and Legoland.
Private equity firm Apax partners created Merlin by backing a £57m management buyout of Vardon Attractions in 1998.
Hermes Private Equity bought the company for £72.5m in 2004 before selling it to Blackstone for £102.5m a year later.
Dubai International Capital and Danish family trust Kirkbi later bought into the business, but the former has since exited its stake.
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