The compromised accounts appear separate from a 2014 breach disclosed in September, when Yahoo revealed that 500 million accounts had been accessed.
Yahoo has taken steps to secure user accounts and is working closely with law enforcement.
The internet giant is also notifying potentially affected users and is requiring people to change their passwords.
If you have fallen victim to fraud, report it to Action Fraud and get a crime reference number.
You can also find a full list of FAQ’s from Yahoo containing details about this issue. If you need further information or assistance with your account, please visit the Yahoo help page where you will find the latest information and may be able to access direct customer support.