Microsoft says it has "rolled back a network change" it thinks may be linked to tens of thousands of worldwide users being unable to access its services, including Teams and Outlook.
Downdetector, which tracks website outages, showed more than 5,000 people in the UK had reported the email service Outlook was inaccessible.
We've rolled back a network change that we believe is causing impact. We're monitoring the service as the rollback takes effect.
Previously, the firm said it had "isolated the problem to networking configuration issues" and was analysing 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.