Mobile network O2 has said it is investigating a technical fault that has left thousands of mobile customers unable to get online.
UK customers have reported not being able to use mobile data to access the internet and other services on the company's network.
It has confirmed engineers are working to fix the issue.
"We are aware our customers are unable to use data this morning," the company said on its official Twitter account.
"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."
The service status page of the O2 website added that "voice calls are working OK" and apologised for any inconvenience caused.
Other mobile networks, including Sky, Tesco and Giffgaff have also been affected by the problem because their networks use O2 services.
O2 said it would share further updates as soon as they were available.
In a further tweet, O2 said the problem was related to a software issue in the system of a third party supplier.
"One of our 3rd party suppliers has identified a software issue in their system. Our technical teams are working extremely hard to resolve this," the company said.
"We'd encourage you to use Wi-Fi wherever you can and we are really sorry."