O2 - which is the second-largest mobile network in the country behind EE - issued a statement saying: "We are aware our customers are unable to use data this morning".
Customers have also been encouraged to use WiFi whenever possible if they need internet access before the problems have been fixed. It also said that the issue could be causing "intermittent problems" when customers make voice calls too. Our technical teams are investigating.
The Financial Times said the O2 outage was related to an issue with software provided by Ericsson.
O2 customers are now experiencing difficulties after the mobile network went down. "We apologise for any inconvenience".
It has 25 million direct customers, but also provides services for the Sky, Tesco, Giffgaff and Lycamobile networks. GiffGaff has confirmed its service is also offline.
TFL said: "The company that updates the data to our network of Countdown Systems is now experiencing difficulties in providing this service".
Some customers have reported being unable to use their data, while others are struggling to send texts.
Writing on Twitter, O2 said this morning: "Outage Update: One of our third party suppliers has identified a software issue in their system". Our technical teams are working extremely hard to resolve this.
The mobile phone provider is now working to address the issues but it's unclear how long it might take before customers can use their data again, with many having to rely on public WiFi to access their emails and social media.