Following a two-day global outage, Skype's internet phone service is back to normal. Skype says the outage was triggered by millions of its users rebooting their computers following a routine download of a Windows update.

About 220 million people across the world use Skype's service, which enables users to make long-distance phone calls over the internet.

The worldwide outage began on Thursday as Skype users from Vietnam to Canada said they could not log on to make calls or send instant messages.

On its Heartbeat blog on Monday, the company said: "The disruption was triggered by a massive restart of our users’ computers across the globe within a very short time frame as they rebooted after receiving a routine set of patches through Windows update."

Skype added that the massive global restarts caused a flood of log-in requests. That combined with the overwhelming network traffic caused a chain reaction resulting in the massive outage.

"Regrettably, as a result of this disruption, Skype was unavailable to the majority of its users for approximately two days.

"The issue has now been identified explicitly within Skype. We can confirm categorically that no malicious activities were attributed or that our users’ security was not, at any point, at risk.

"Skype has now identified and already introduced a number of improvements to its software to ensure that our users will not be similarly affected in the unlikely possibility of this combination of events recurring," said the company on its blog.

Besides making computer-to-computer calls, Skype also connects cellphones and traditional land line telephones.

With files from the Associated Press