Bosses at Britain's biggest three supermarket chains have banned office staff from accessing social networking site Facebook while at work. Senior executives at Tesco, Asda and Sainsbury's have ordered IT teams to block access to the site as part of measures to ensure employees don't waste time surfing the net for fun when they should be working. Facebook, which allows people to create a web page about themselves in seconds and exchange messages with other members, has become a global phenomenon with 34 million users worldwide. In the UK alone, more than two million people have signed up to the site since it went live in February 2004. But big retailers are worried about the effect the site could have on staff output, with research showing that more than half of active users return to the site daily and spend an average of 20 minutes on it every visit. The site is said to be so addictive that it has been nicknamed 'Crackbook'. This week Tesco insisted that blocking the site was "not a Draconian measure". "Facebook has been blocked because more people are becoming aware of it," said a spokesman. "It is normal business practice to block access to non-business related material online." But the measure was criticised by Mike Emmott, employee relations adviser at the Chartered Institute of Personnel and Development. "If an employer has got evidence that a particular site is going through work places like wildfire, and that people feel an irresistible urge to go online in work time, then employers are well within their rights to ban that site," he said. "But banning specific sites may create an impression that the management doesn't trust people to use their time at work focusing on work issues, and could be bad for morale. And if you have to do this to stop staff wasting time what does that say about the line management's ability to supervise people?" This week, a survey conducted by software company Sophos found that half of all workers in the US were blocked from using the site on office computers.