The telecoms provider says its technical teams are working to fix the issues, which is also affecting Giffgaff, Sky Mobile and Tesco Mobile, which piggyback on its network. Check our Piggybacking guide for full info on which network really provides your signal.
While O2 initially said voice calls and texts were still working, some users have reported losing signal entirely.
Customers have flooded social media to report the outage, with O2’s Twitter account saying that data should be up and running again “asap”. In the interim, users will have to connect to Wi-Fi.