HMRC users have complained that the platform is down this morning and they are unable to access their accounts.
Problems started being reported just before 8am this morning, with more than 200 issues being reported.
Issues appear to be ongoing. HMRC has been approached for comment.
Downdetector is an online website that allows users of popular services including X (Twitter), Instagram, Facebook, Virgin Media and more to report if they are having trouble with the services.
The site only reports an incident when the number of problem reports is significantly higher than the typical volume for that time of day.
Describing itself on its website, Downdetector is "where people go when services don't work."
The website monitors disruptions to various key services including the internet, web hosting platforms, banks, social media and more.
You can see the full methodology and learn more about how Downdetector collects status information and detects problems via its website.
In short, users can get involved by submitting any issue reports on the platform.
Problem indicators are also collected from social media and other factors on the web, Downdetector explained.
These reports are then validated and analysed in real-time.
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An incident is then detected if the number of reports is significantly higher than the typical average for that particular service.
When a user goes to investigate the issue, they can see what the most reported problems are currently and check if their areas are in one of the outage hotspots.