The company say they’re investigating with "high priority" after its data services went down, with tens of millions of users potentially affected, included those across Oldham, Rochdale and Tameside.
The problem is also affecting customers of Sky Mobile, Tesco Mobile, Lyca Mobile and GiffGaff - which all use O2's infrastructure.
O2 have tweeted "Our teams are working closely with one of our 3rd party suppliers who has identified an issue in their system which has impacted mobile services globally. This is our top priority and we are really sorry."
Customers are currently unable to access 3G or 4G services, which means text messages, internet access, picture messages and some apps are unavailable.
O2 insist that regular voice calls are not affected, though customers have complained of a lack of service on social media.
It's understood around 32 million customers have been affected across the UK and Europe.
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