Mobile network operator O2 left thousands of customers without a connection throughout most of Thursday after it experienced its third network outage in the space of a year.

The 12-hour outage affects O2 customers across the country and brought mobile data, text messaging and call services to a standstill.

O2 confirmed the issues on its website, revealing that the problems began at 10:45am yesterday and lasted until 11:45pm.

Although the outage was less severe than previous incidents, a number of O2’s customers took to social media sites like Twitter to vent their frustration.

One said, “#O2 outage again! Love my iPhone, hate O2! No unlimited data packages COME ON PEOPLE!”

Another complained saying: “Seriously unimpressed with @O2 today. Tell you what, guys - you're going to have to go some to keeping my business after yet another outage.”

O2 explained the reason for the outage on its website. The operator said: “Routine upgrade work on our 2G and 3G network led to unexpected congestion affecting around one percent of O2 customers."

"We apologise to those customers and thank them for their patience,” it added. 

The news follows a similar outage that occurred in July 2012 and forced O2 to compensate many of its customers who were cut off for more than 17 hours. Another outage occurred in October 2012, reportedly affecting two million people.

Last week, the company activated its 4G network in five new UK cities – Birmingham, Coventry, Leicester, Nottingham and Sheffield – giving customers access to superfast data speeds on their mobile and tablet devices.

EE and Vodafone have also switched on their own 4G networks and Three plans to launch 4G services with unlimited data packages in December.

O2 tries to drum up business on its website by boasting about its "award-winning network" and 200 engineers "dedicated to network performance".