Microsoft is working to resolve an issue which has taken its Outlook email service offline, leaving users unable to access their communications.
Those attempting to log-in to their email accounts have been met with a "service unavailable" error message.
The technology giant's Outlook.com platform began reporting issues this morning and Microsoft has confirmed it is investigating the cause.
A message on the company's service status website said the fault has left users unable to access emails, and it is "collecting additional data from the affected infrastructure to aid in our investigation to determine the cause of impact".
In a further response to users on Twitter, Microsoft apologised and said its engineering team is "aware and is actively investigating" the problem.
The popular email platform supports Outlook, Hotmail and Windows Live email accounts and is estimated to have more than 400 million users, although the number of those affected by the outage has not been confirmed.
Many users have taken to social media to express frustration at the incident, which has left people unable to access their work emails at a time when large numbers are working from home and rely on emails to communicate with colleagues.
According to web service monitoring platform DownDetector, users across Europe, as well as parts of Africa and India, have reported problems with Outlook.
It said issues with Outlook were first reported at around 7am this morning and since then more than 3,000 reports from users struggling to access the platform have been made to DownDetector.