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A major outage was affecting Yahoo! email users worldwide.
At least 1,400 people - including several hundred in the UK - are believed to have been impacted by the issue, which was confirmed by the company just after 6pm.
Users reported being unable to send, receive or access any mail.
Customers of Sky are among those who were unable to access their accounts.
Yahoo! said on its Twitter account: "You may not be able to access Yahoo Mail at the moment.
"Our top priority right now is getting this fixed. We appreciate your patience during this time."
The company later added: "We apologise for the inconvenience you are experiencing.
"At this time, we have identified the issue and it is being addressed.
"While we cannot provide a specific time when the issue will be resolved, our team is working to correct it as soon as possible."
The company confirmed the issue had been resolved just after midnight.