O2 is experiencing a UK–wide service outage, with customers unable to access data via their smartphones.
The network-wide problem surfaced in the early hours of Thursday 6th December, with some of the network's users saying that they were also unable to send text messages.
Those on O2–backed networks, such as Tesco Mobile and Giff Gaff, have also been affected.
O2 said: “We are aware our customers are unable to use data this morning. Our technical teams are working on the issue with high priority.
"We are really sorry and working as hard and as fast as we can to fix this.”
At 9.30 am, it posted a further update to Twitter: “One of our 3rd party suppliers has identified a software issue in their system.
"Our technical teams are working extremely hard to resolve this. We’d encourage you to use Wi-Fi wherever you can and we are really sorry.”
O2 customers have voiced their displeasure on Twitter.
Commenting on O2’s mobile data outage, Ernest Doku, mobiles expert at uSwitch.com, said: "O2 users affected by this mobile data outage will understandably be concerned and frustrated.
"While it's positive that voice calls are still up and running, without a projected timeframe for a fix, this is likely a worrying situation for a large proportion of O2's some 32 million UK customers.
"Customers who are suffering can keep a track of this incident using O2's own network status checker and ask the provider to keep them posted.
"In the meantime, O2 customers can still place and receive calls, but will have to resort to using WIFI if they want to get online.
"For the millions of users who are out and about and rely on smartphone maps to get around, it's worth considering that apps like Google Maps allow customers to download maps on WIFI and view them offline.
"With little idea of when this problem will be sorted, it's worth preparing before heading out to make sure you're not caught out by this data downtime."