An apparently major outage on O2's mobile network on Wednesday has left large numbers without phone or data service and the company on the receiving end of a storm of criticism from irate users.

The number of the company's estimated 23 million UK users affected by the problem is still a mystery but judging by the number of Twitter comments (sent from PCs, not mobiles) the numbers must be many tens of thousands at least.

As of 21.30 BST, the majority complained of not having been able to use their handsets 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 plaintive messages posted on the company's Twitter account.

As much as the service going down, many users were left angry by the lack of information from O2 as to when their service might return.

“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," was as far as O2 would go.

It's not been a great period for mobile networks and the demands being placed upon them.

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.

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