Internet users in the UK and United States have been unable to log in to their Yahoo email accounts after an underwater cable was cut during maintenance work.
A notice on the help page of Yahoo's website said: "We are aware that Yahoo Mail is slow or inaccessible for some of our users. The issues were a result of an underwater fibre cable cut, caused by a third party while fixing a separate cable.
"The engineering team has rerouted email traffic to mitigate accessibility issues. A cable repair ship has been mobilised and will be at the site this weekend. We apologise for the inconvenience as we certainly understand email is a critical service for our customers."
The notice added: "Some Yahoo Mail users are experiencing delays in accessing and viewing their mail. We'll be posting updates regularly on this page to keep everyone informed of our progress. Thank you for your patience as we work to fully resolve this."
Details of the issue first appeared when the firm said on Twitter that there was an issue.
The news comes a day after Yahoo was confirmed as the new default search engine for Mozilla's Firefox web browser, the world's third most popular, in a deal which sees them replace Google.
According to analytics firm ComScore, Yahoo Mail, which is the service most troubled by the cable cut, had 298 million users worldwide at the last count in late 2012. The worst affected regions as a result of the cut appear to be western Europe and the eastern United States.
Reports of problems surged in the UK this morning, with the number of users logging on to report an issue rising from less than 20 to more than 1300 in two hours.