Problems with the service were first flagged at 7am on Thursday as users reported they were unable to log in to their accounts, or that emails were slow and unresponsive.
Customers with TalkTalk, Sky and BT were also affected by the outage as their service is powered by the US brand.
Yahoo’s customer care team said in a series of tweets that they were addressing technical issues with the service.
“We’re working diligently and will do our best to keep you in the loop as we work to address technical issues with accessing some of our services,” one tweet said.
“We will be posting as soon as an update is available and thank you for your patience!”
According to Downdetector, issues began at 7am and were affecting around 5,000 users an hour later.
Sky, TalkTalk and BT also tweeted to apologise for any inconvenience caused to their customers and that technical teams were working to solve the issues.
A spokesman for Verizon Media, Yahoo’s parent company, said: “We are aware of a technical issue that is impacting our services.
“We’ve been able to get most services back online already and appreciate our customers’ patience as we work to get everything up and running as soon as possible.”