Ericsson, whose central user database caused O2's 2012 25-hour mega-outage, told The Reg: "We are aware of the issue and are working together with our customers to solve it as soon as possible".
O2 is the second largest mobile operator in the United Kingdom; the company says it has around 32 million connections to its network and also hosts virtual network operators, such as Sky Mobile, Lyca Mobile and giffgaff.
"Voice calls are working OK. we are sorry for any inconvenience".
The company said one of their third party suppliers has identfied a software issue in their system.
"Our technical teams are working with their teams to ensure this is fixed as quickly as possible".
Australian cricketer's brother nabbed over terror charges
Police in Australia have charged Arsalan Khawaja , 39, with forgery and an attempt to pervert justice. Following "definitive advice", NSW Police formally withdrew the charges on November 16.
An O2 spokesperson said the network was continuing to investigate the problems.
Some customers have reported being unable to use their data, while others are struggling to send texts.
The problems are UK-wide and O2's Twitter account spent the morning engaging with customers who were having issues with data. GiffGaff has confirmed its service is also offline. We believe other mobile operators around the world are also affected.
Tens of millions of smartphone users in Britain lost internet access today after a software glitch brought down O2's data network.
It's encouraging users to "use Wi-Fi wherever they can". "Our technical teams are working extremely hard to resolve this", the company said. Our technical teams are investigating.