A significant outage on O2's mobile network left large numbers without phone or data service on Wednesday, sparking a storm of criticism from irate users.

An estimated 23 million UK users could have been affected by the problem. The cause is still unconfirmed but judging by the number of Twitter comments the numbers affected are in the tens of thousands at least.

As of 21.30 BST, the majority of Tweets were about not being able to make or receive calls for six to eight hours.

“Our engineers are doing their level best to resolve the current network issue. Apologies for the inconvenience caused,” read one of several messages posted on the company's Twitter account.

“The problem is an issue within part of our core network that is preventing some mobile phones from successfully connecting. The problem is not location-specific. All possible resources across our and our suppliers' engineering teams are being deployed to restore service as soon as possible," O2 said.

On 9 July, it was the turn of Orange whose 26 million French customers lost the ability to make calls, disruption that lasted until the following morning.

Another big network blackout was suffered by BlackBerry last October in various parts of the world that severely dented the company's already failing reputation.