Steve Ballmer, the outgoing chief executive of Microsoft, has acknowledged that under his leadership the company became too focused on Windows computers and missed the boat on smartphones.
Mr Ballmer said Microsoft had almost "no share" in the smartphone market as a result of the decision.
"I regret that there was a period in the early 2000s when we were so focused on what we had to do around Windows that we weren't able to redeploy talent to the new device called the phone," he told analysts at a meeting in Washington.
"That is thing I regret the most," he repeated, adding "It would have been better for Windows and our success in other foreign factors."
Windows' operating system has a global market share of just 3.7pc, according to market intelligence firm IDC, putting the firm in a distant third spot behind Google's Android operating system, which has a market share of 79.3pc, and Apple's iOS, with a 13.2pc share.
However, Windows has grown its market share by 77.6pc over the past year, helping the company to overtake Blackberry and Symbian - Nokia's former platform of choice - in terms of market share.
Mr Ballmer, who will step down within the next twelve months, said Microsoft had significant "upside opportunities" to expand its mobile division.
"We have the tools. There's economic upside here. In the long run, we are almost uniquely poised to seize the opportunity," he said, in a typically high-volume presentation.
"Today I'm speaking as an investor. You all own Microsoft stock, cheer for it, for God's sake."
Microsoft snapped up Nokia earlier this month in a €5.44bn (£4.61bn) deal that the computing giant hopes will increase its share of the smartphone and tablet market through faster innovation.
Mr Ballmer's replacement is yet to be announced, although Nokia chief executive Stephen Elop is hot favourite to take the top job.