Faults were reported as early as 4am and customers are still having problems using data, sending texts and making calls.
The company said one of their third party suppliers had identfied a software issue in their system. Work is currently ongoing to fix the fault and customers are being encouraged to use wi-fi where possible.
Companies that share O2's network – such as GiffGaff, Sky Mobile and Tesco Mobile – are also being hit by the problems. But what are your rights about claiming compensation?
Ofcom say that depending on the circumstances, it may be appropriate for your provider to offer you some money back while repairs are being carried out.
In more extreme cases, where repairs take much longer (for example it takes longer than usual to access a mast site to undertake repairs), you may be entitled to an additional refund or account credit.
In cases where you have been without service for some time, you may also have the right to leave the contract without penalty. There may be a term in your contract saying you can do this if your provider has failed in its obligations to you or breached a key condition.
What have O2 said?
The company have so far not said anything about whether they would be giving customers compensation for the outage.
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.
How do I complain?
Ofcom says that if your provider fails to repair a fault by the date promised, or you are unhappy with how long it is taking, you should follow their formal complaints procedure. Details should be available through their website or customer services.
If your problem is still unresolved after eight weeks you can submit your complaint to an independent Alternative Dispute Resolution (ADR) scheme.
If your problem cannot be resolved, ask your provider for a ‘deadlock’ letter so that you can refer your dispute to the relevant ADR scheme directly before the eight week mark.
Ofcom has approved two ADR schemes - CISAS and Ombudsman Services: Communications.
For more information, see the Ofcom website.