The chief executive of O2 UK, Ronan Dunne has tweeted:
To all our affected customers - I'm very sorry. The network is back. My focus now is restoring your confidence and trust in O2.
Thousands of disgruntled O2 customers took to Twitter this morning to vent their frustration at still being unable to use their phones.
Thousands of customers face a second day of disruption with O2's mobile network, despite reassurances that it has been partially restored.
Thousands of O2 customers across the country were still unable to use their mobile phones today.