02 says it is investigating with high priority after users have been reporting that data services on the network are not working.
The network said: "Our technical teams are investigating reports of issues when using data.
"Voice calls are working OK. Our technical teams are investigating. We apologise for any inconvenience…"
Customers tweeted that they were unable to use 3G and 4G services.
Stacey Caine posted: "[email protected] come on 02 unable to text anyone all my texts are failing, and no data. Currently using my WiFi."
Andrew Davies said: "Where has all the data gone…[email protected] appears to have broken this morning! No mobile access on my phone Good job I have different provider on iPad!"
@anthony_vol1 tweeted: "Nice start to today 02 data has gone down, thank God for free Wifi on buses."
An 02 tweet said: "We are aware our customers are unable to use data this morning. Our technical teams are working on the issue with high priority.
More from Science & Tech
"We are really sorry and working as hard and as fast as we can to fix this. Please keep an eye on our status checker: http://o2.uk/ServiceStatus ."