Vodafone says it is working to tackle "disruption" to its mobile and fixed-line broadband services.
It has experienced a fault with an international link used to transmit data between countries but is rerouting traffic to address the issue.
Reports on the DownDetector website indicate that subscribers in the UK, Ireland, Greece, Italy, Portugal and Germany are among those affected.
The problem appears to have begun after 14:00BST.
Some affected users have, however, reported that issues with email and other internet-based activities have been resolved.
The fault comes a day after the firm announced that it had begun allowing users to link their accounts to Amazons Alexa service, letting them call contacts via the US companys smart speakers. However, there is no suggestion that this was the cause of the outage.