The problems mean that the companies that share its network – such as GiffGaff, Sky Mobile and Tesco Mobile – are also being hit by the problems.
Calls are still working, O2 said. But data connections are broken. Affected phones, which may be every single O2 device, simply fail to show any kind of data connection and will not connect even when turned off and back on or when other workarounds are tried, according to users experiencing the problems.
Many users will have received an alert to their handsets warning of the drop in service. Some users also suggested that SMS messaging might not be working, too, leaving them unable to send texts.
"Our technical teams are investigating reports of issues when using data," O2 wrote on its status page. "Voice calls are working OK. Our technical teams are investigating. We apologise for any inconvenience. Updates will be shared here as soon they are available."
That page was also having issues at the time of writing and was intermittently failing to load.
The outage tool - which allows customers to check if their area is affected - was also down.
The problems are UK-wide and O2's Twitter account spent the morning engaging with customers who were having issues with data.