Thousands of users around the world are reportedly unable to access services by Microsoft Office, including Outlook and Teams.
Microsoft has said it is investigating the matter and has identified a potential networking issue.
In a series of Tweets seen by millions, the company said it has rolled back a network change that it believes is responsible for the outage.
According to Downdetector, which monitors website outages, more than 5,000 people in the UK had reported that Outlook was inaccessible. Users around the world have also reported similar issues.
Other Microsoft services affected by the outage include Xbox Live and OneDrive for Business.
National Technology News has approached Microsoft for comment.
Microsoft recently became the latest technology firm to announce job cuts. The company will lay off around five per cent of its workforce – about 11,000 roles, mainly from human resources and engineering divisions.
The news comes six weeks after Amazon suffered an outage for just over an hour, causing some customers to experience issues with the shopping site.
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