British mobile telecoms operator O2 said a software issue at a third-party supplier caused a data outage today and it was working to fix the issue "as quickly as possible".
O2, which is owned by Spain's Telefonica, said customers of Tesco Mobile, Sky Mobile and GiffGaff, which all use its network, were also affected by the outage.
O2 said other operators around the world were affected, but did not identify any companies.
Users of O2 services, who were unable to access the internet or apps that require an internet connection, poured onto social media to complain about the outage.
The UK's second-largest mobile operator initially said in a tweet that it was looking into issues with data and voice service usage.
It later said in a separate tweet that voice calls were working properly.
"One of our third party suppliers has identified a global software issue in their system which has impacted us," O2 said in a statement.
"We believe other mobile operators around the world are also affected."
SoftBank Group's mobile phone services were disrupted in some parts of Japan today, but it was unclear whether its problems were related to those of O2.