Research by the job-hunting website revealed, found that a third of those that access social networks during the day do so for more than 30 minutes. Furthermore, six percent admitted they spend over an hour a day on social networks.
MyJob.co.uk estimated that time spent on social networks is costing the UK economy up to £14bn in lost productivity.
However, only 14 percent of office workers said they were less productive as a result of social media, while 10 percent claimed Facebook and Twitter has made them more productive.
One third of Brits believe access to social networks such as Facebook and Twitter should be barred during office hours
Lee Fayer, managing director of Myjobgroup.co.uk, said: "Our results clearly show that UK workers are spending increased time whilst at work on social media networks, which, left unchecked, could have negative repercussions on the productivity of many companies across the country.
"While we're certainly not kill-joys, people spending over an hour per day in work time on the likes of Facebook and Twitter are seriously hampering companies' efforts to boost productivity, which is more important than ever given the fragile state of our economy."
Fayer advised companies to monitor use of social networking sites during work hours and ensure that employees are not abusing their freedom of access to these sites."