Tens of thousands of users around the world have reported being unable to access Microsoft services, including Teams and Outlook, the BBC reports.
Downdetector, which tracks website outages, showed more than 5,000 people in the UK had reported the email service Outlook was inaccessible.
Other services including Teams and Xbox Live were also reported by users as not working.
Microsoft says it is investigating the outage.
In a statement, the firm said it had “isolated the problem to networking configuration issues” and was analyzing the “best mitigation strategy to address these without causing additional impact.”
Microsoft also said it was investigating a connectivity issue with its cloud computing service, Azure, affecting what it called “a subset of users.”
Microsoft Teams is used by more than 280 million people globally, primarily in businesses and schools, where it can be of critical importance for calls, meetings and general service organisation.
According to Downdetector, issues have been reported in many other countries around the world, with thousands of reports in India and Japan alone.
Microsoft has been approached for comment.