O2 - which is the second-largest mobile network in the country behind EE - issued a statement saying: "We are aware our customers are unable to use data this morning".
"Our technical teams are working on the issue with high priority".
O2 has 25 million customers, but supplies network coverage to Sky Mobile, Tesco, Giffgaff and Lycamobile, leaving a total of 32 million reliant on its coverage.
Outages were reported across the United Kingdom, including Manchester, London and Exeter.
The outage also has serious implications for places that rely upon the O2 network for real-time informational services, as reported by the Independent.
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. Londoners, for example, are finding that the Countdown timers at bus stops are all crippled as they can't communicate with London's transport executive, TfL.
O2 said that "one of our third party suppliers has identified a global software issue in their system". "Our technical teams are working extremely hard to resolve this", the company said.
"Updates will be shared here as soon they are available". "We'd encourage you to use Wi-Fi wherever you can and we are really sorry".