Thousands of customers are believed to have been affected in the data blackout, which is also affecting customers of other mobile providers who use O2's service.
However, some customers reported that this was not the case.
OUTAGE UPDATE 10.45am: One of O2's third party suppliers has identified a software issue in their system. It also said that the issue could be causing "intermittent problems" when customers make voice calls too.
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. "We are really sorry and working as hard and as fast as we can to fix this".
O2 customers across the United Kingdom have been hit by a network outage that has left them unable to use 4G and data services, meaning they can't access the internet without a Wi-Fi connection. It also affected services that use O2's network, bringing down digital timetables at London bus stops... The company estimated 32.1 million people use its mobile network in the UK.
TFL said: "The company that updates the data to our network of Countdown Systems is now experiencing difficulties in providing this service".
He said: "We're aware that our customers are unable to use data this morning".
Many users took to Twitter to complain about the difficulties, which were first reported at about 05:30.
"Updates will be shared here as soon they are available".