Skype said Monday the cause of a two-day service outage was caused by an unusually high number of restarts by users who had downloaded a Microsoft Windows security patch.

    The Internet phone company said on its Web site that the restarts “combined with the lack of peer-to-peer network resources, prompted a chain reaction that had a critical impact.” That unveiled a bug in the system which didn’t allow its network to “self heal,” the company said.

    Skype said the problem was internal, there were no outside malicious activities and users’ security was never at risk.

    The outage occurred on Thursday and service was restored Saturday. It was the worst outage in the 4-year-old company’s history.