Microsoft cofounder Bill Gates said his "greatest mistake ever" was allowing Google to develop Android and not creating a successful smartphone operating system himself.

"There's room for exactly one non-Apple operating system," he said, "and what's that worth? $400 billion that would be transferred from company G [Google] to company M [Microsoft]."

Microsoft phased out its Windows-powered smartphones in 2017 after they failed to make a significant impact on the market.

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Bill Gates has said that his biggest mistake at Microsoft was allowing Google to develop Android — and it cost the company $400 billion.

"The greatest mistake ever is the whatever mismanagement I engaged in that caused Microsoft not to be what Android is, [meaning] Android is the standard non-Apple phone form platform. That was a natural thing for Microsoft to win," Gates said last week.

The billionaire made the comments in a conversation with Eventbrite cofounder and CEO Julia Hartz a at an event hosted by venture capital firm Village Global. The news was first reported by TechCrunch over the weekend.

Windows phones were wound down in 2017. YouTube/Austin Evans

"It really is winner take all," he said. "If you're there with half as many apps or 90% as many apps, you're on your way to complete doom. There's room for exactly one non-Apple operating system, and what's that worth? $400 billion that would be transferred from company G [Google] to company M [Microsoft]."

Read more: Microsoft's Windows Phone platform is dead

Microsoft's Windows-powered phones failed to make a significant impact on the smartphone market and enjoy the same success as Android or iPhone. This was partly because of the problems it had with its app store, specifically, retaining apps once they had been developed. Its Windows phones launched in 2010 but were phased out by 2017.

Gates, who remains a board member at Microsoft, added: "We are a leading company. If we got that one right, we would be the company. But oh well." Microsoft is currently the most valuable company in the world, with a market cap of $1.05 trillion, but it vies for top spot with Apple and Amazon.

You can watch the full interview with Bill Gates here: