Thousands of users around the world have reported that they are unable to access Microsoft services, including Teams and Outlook.
Downdetector, which tracks websites, showed more than 5,000 people in the UK had reported that email service Outlook was inaccessible.
Other services including Teams and Xbox Live were also reported as being non-functional.
Microsoft said it is investigating the outage.
Microsoft Teams is used by more than 280 million people globally, primarily as a critical part of businesses and schools, where it is used for calls, meetings and organisation.