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.
Even though you many not be on O2 yourself, this could still seriously affect your morning, because lots of connected devices rely on communication over the network.
"Our technical teams are working on the issue with high priority".
The company has confirmed that voice calls are still working, and have asked customer to alert them to any network issues they may be experiencing.
"We'd encourage you to use Wi-Fi wherever you can and we are really sorry".
In a further tweet, O2 said the problem was related to a software issue in the system of a third party supplier.
Calls are still working, O2 said.
O2 customers across the United Kingdom have been hit by a network outage that has left them unable to use 4G and data services, meaning they can't access the internet without a Wi-Fi connection.
An O2 spokesperson confirmed the problems, saying: "Our technical teams are investigating reports of issues when using data". Our technical teams are working extremely hard to resolve this. "Updates will be shared here as soon they are available". The outage tool - which allows customers to check if their area is affected - was also down.