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 said: “One of our third-party suppliers has identified a global software issue in their system which has impacted us.”
The spokesperson added: “We believe other mobile operators around the world are also affected. Our technical teams are working with their teams to ensure this is fixed as quickly as possible.
“We’d encourage our customers to use wi-fi wherever they can and we apologise for the inconvenience caused.”
Voice calls are unaffected by the problem.
The outage is 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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