O2 said: "Our technical teams are investigating reports of issues when using data".
O2 is the second largest mobile operator in the United Kingdom; the company says it has around 32 million connections to its network and also hosts virtual network operators, such as Sky Mobile, Lyca Mobile and giffgaff.
The service status page of the O2 website added that "voice calls are working OK" and apologised for any inconvenience caused.
In a tweet, the network said that the issue lay with one of its third-party suppliers, and recommended that users switch to Wi-Fi wherever possible.
"Our technical teams are working with their teams to ensure this is fixed as quickly as possible".
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.
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As a result users have been experiencing issues with 3G and 4G services, with others unable to make calls or send texts.
"Updates will be shared here as soon they are available".
Other mobile networks, including Sky, Tesco and Giffgaff have also been affected by the problem because their networks use O2 services.
When these networks are taken into account, up to 32 million customers are affected.
O2 customers are now experiencing difficulties after the mobile network went down.
Transport for London's live bus timetables rely upon the O2 network. Our technical teams are working extremely hard to resolve this.