An additional seven million customers of Sky, Tesco, Giffgaff, and Lycamobile, which O2 provides services for, are also impacted.
The problems seem to have started at around 8:00 AM this morning and are ongoing at the time this article was posted.
A third-party supplier to the company had identified a "global software issue" in their system, which primarily affected data services.
The technical problem left millions of workers without access to emails and news on their morning commute and enraged those who wanted to access social media.
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With O2 United Kingdom and Softbank customers being thrown back to the 90s after a network outage, reports have emerged suggest Ericsson was the root cause. The service had not resumed by the afternoon.
We are aware our customers are unable to use data this morning.
O2 issued the following statement on its website: "Our technical teams are investigating reports of issues when using data".
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The network confirmed on Twitter that its users are "unable to access data services" and 4G coverage due to a massive data failure.
Earlier, O2 said it suspended planned work or updates to its systems and network. Our technical teams are investigating. "We are really sorry and working as hard and as fast as we can to fix this".
However, when the network was down for seven hours in 2015, the company did not offer compensation to all customers, instead saying payouts would be assessed on a "case-by-case" basis.
London Bus electronic timetables, which rely on the O2 network, have also been affected.
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Engineers are working to fix the issue that has meant customers can not use their mobile data to access the internet and other services on its network.