O2 is investigating with "high priority" as its data services remain out of action, with tens of millions of users potentially affected.
The problem has also hit customers of Sky Mobile, Tesco Mobile, Lyca Mobile and GiffGaff - all of which use O2's infrastructure.
The problem started early this morning and has still not been fixed, with customers unable to use 3G and 4G services to access the internet.
O2's latest statement said resolving the issue remained "a top priority for us and we are working as quickly as possible to fix the issues".
It added: "All O2 technical teams are working closely with one of our third party suppliers who has identified a global software issue in their system which has impacted data services.
"As a result of this incident, we are experiencing a high demand on our network which means some customers may have issues making and receiving voice calls.
"We believe other mobile operators around the world are also affected."
The firm's boss, Mark Evans, tweeted his apologies and said were doing everything possible to sort things out.
"My teams are working really hard with Erricson to find a swift resolution," he said.
Social media users have been venting their frustration.
Stacey Caine posted: ".@02 come on 02 unable to text anyone all my texts are failing, and no data. Currently using my WiFi."
Andrew Davies said: "Where has all the data gone.....@02 appears to have broken this morning! No mobile access on my phone Good job I have different provider on iPad!"
@anthony_vol1 posted: "Nice start to today 02 data has gone down, thank God for free Wifi on buses."
"Me, my mum and my sister don't have any mobile data either, we have 30gb showing on our 02 account!! Definitely think they have gone down," tweeted @stephanie_bint.
Nicola Barton said she was heading to Great Ormond Street Hospital when she realised data was down.
"Out all day at GOSH for appointments for my daughter and 02 decide today is the day for its data service to go down. Fabulous, will rely on intermittent connections to wifi for the day then."
Swedish telecoms company Ericsson said it was aware some of its mobile operator customers were having problems and that it was working with them to "solve it as soon as possible".
In London, the electronic timetables at the city's bus stops were also reported to be out of action - thought to be related to the O2 outage.