Both Tesco Mobile and Giffgaff run on the O2 network which means that they are experiencing the same issues as O2 customers.
O2 users are not able to access data and 4G services on their phones, with problems reported since 4.45am this morning.
O2 is investigating reports of issues when using 3G or 4G data, adding that "voice calls are working OK". Work is now ongoing to fix the fault and customers are being encouraged to use wi-fi where possible.
When these networks are taken into account, up to 32 million customers are affected.
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It said: "This allowed hundreds of people from both our technical teams and our third-party supplier's teams to focus on fixing the problem, helping us to restore data services as quickly as we can which will also help reduce the high network demand we're now seeing for voice calls". Londoners, for example, are finding that the Countdown timers at bus stops are all crippled as they can't communicate with London's transport executive, TfL.
O2 did not say what is causing the fault. "This issue is being investigated".
O2 have assured customers that they are working as hard as possible to get the networks up and running as soon as possible.
Calls are still working, O2 said.
It is unclear whether the O2 outage, said to be the result of a "global software issue", is linked to the problems with Angus Council's parking meters. "Ericsson sincerely apologises to customers for the inconvenience caused".