A statement on the O2 website said: "Our technical teams are investigating reports of issues when using data. We apologise for any inconvenience".
O2 said that "one of our third party suppliers has identified a global software issue in their system".
Users across the country said they were unable to access the internet without a wi-fi connection.
If you spent this morning freaking out you'd used up your data allowance and tried desperately to call O2 to give them a piece of your mind, you're not alone.
The issue is also impacting customers of MVNOs that piggyback on O2's infrastructure, like GiffGaff, Tesco Mobile, and Lyca Mobile.
Thousands of O2 customers in the United Kingdom have reported problems using data and 4G services. Transport information services have also been affected.
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The outage is thought to be affecting about 32 million in the UK.
Some users also suggested that SMS messaging might not be working, too, leaving them unable to send texts. We believe other mobile operators around the world are also affected.
O2 has confirmed problems with mobile data across its network this morning (Thursday, December 6, 2018).
O2 says it is trying to resolve the issue as fast as possible.
"Our technical teams are working with their teams to ensure this is fixed as quickly as possible".