Thousands of O2 customers in the UK have reported problems using data and 4G services.
Many users took to Twitter to complain about the difficulties, which were first reported at about 05:30.
Users across the country said they were unable to access the internet without a wi-fi connection.
An O2 spokesperson confirmed the problems, saying: "Our technical teams are investigating reports of issues when using data."
The spokesperson added: "Voice calls are working OK. We apologise for any inconvenience."
The outage is also having knock-on effects for other services that rely on the O2 network, including Transport for London's electronic timetable service at bus stops, which has stopped working.
O2 is the second-largest mobile network in the country, behind EE.
It has 25 million direct customers, but also provides services for the Sky, Tesco, Giffgaff and Lycamobile networks.
Once those are taken into account, up to 32 million customers are affected.
Meanwhile, BT Mobile customers are suffering a second day of problems, with many customers unable to send or receive text messages.
"Phone calls and all data mobile services are unaffected," said BT in a tweet.
"Sorry for any inconvenience caused and we hope to have the issue fixed soon."
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