Millions of people are facing a communications blackout today after Microsoft Outlook was hit with a "glitch".

More than 400 million people across the world use Microsoft Outlook as their primary email service, with businesses often choosing it over Gmail due to its integration with other products from the company like Teams and the Office suite.

The scale of the blackout today is currently unknown, but reports have emerged of warnings being sent to users telling them their licences are no longer valid, while others are being told their email addresses and passwords are incorrect. The issue emerged on social media, where customers - who know their bills are paid and passwords up to date - shared their fury over the glitch.

READ MORE: Microsoft Outlook down LIVE: Hotmail email down for millions of people

One person took to Reddit to ask: "Is anyone else getting a constant notification for credentials needed to be signed into their Hotmail or Outlook?

"I keep getting the same notification, putting in my password and it is not accepting it. Just wondering if there's an outage or it's down somewhere because loading even takes too long of the time when I just used it."

Here's everything we know about the outage...

Millions of Microsoft users in all corners of the globe have found themselves unable to log in to the service this morning. Users in some countries, like Italy, say they're facing a total blackout as they're locked out of both PC and mobile services.

As it stands, issues have been reported in Australia, Brazil, Canada, Denmark, Germany, Hungary, Ireland, India, Italy, Peru, Spain, the Netherlands, the UK and the US.

According to outage map, DownDetector, the issues in the UK have been detected in several cities, including London, Manchester, Birmingham, Newcastle and Glasgow.

Reports from disgruntled users claim that both the Outlook app and web services have been affected by the outage. Locked out, people have spent more than an hour trying to log in so they can secure their information.

As they try to log in, users are being hit with a pop-up message that reads: "Connection to server failed."

One user said: "Holy hell, I spent an hour trying to log in and re secure my security, to come here and read that everyone is having this problem!! This happens way to many times!"

While both services are experiencing problems, the majority of issues are with Outlook accounts (53%), according to Downdetector. An additional 37 per cent of users are struggling to login. Eleven per cent have a poor server connection.

Customers are reporting a number of different error messages relating to different access issues.

Some have been seeing pop ups that say "connection to server failed", while others are being hit with 500 and 401 errors.

A 500 error indicates the server is encountering an unexpected condition that prevents it from fulfilling the user's request. A 401 is shown when the request has been unsuccessful due to missing or invalid authentication credentials

One user wrote on X: "I can’t access my Outlook.com email. I keep getting 500 and 401 errors on every device and browser. Microsoft support tools are asking me to log in, but I can’t access my account."

The issues came to public attention when a huge number of posts were shared on social media asking fellow Microsoft users if they were experiencing similar issues.

Many took to forum site Reddit, while others sought assurance on Twitter.

Hours into the widespread disruption, Microsoft remains silent on what's causing the issues. Many have complained of the lack of communication and clarity.

One upset user took to Twitter to say: "Impossible to access my account for hours. I need a solution, please!"

Another replied: "I can't either, and it's the only one I have for work".