Twitter and other social networking websites cost UK businesses more than £1.38bn a year in lost productivity, a new study has suggested.
A survey of nearly 1,500 office workers found 57% used the websites during the day for personal use.
The research, commissioned by IT services and technology company Morse, showed the employees spent about 40 minutes a week viewing the pages.
Based on the average wage of an office worker, this cost firms £1.38bn a year, Morse said.
"The popularity of social networking sites such as Twitter and Facebook has grown considerably over the last couple of years. However, with it has come the temptation to visit such sites during office hours," said Philip Wicks, a consultant at Morse.
"When it comes to an office environment, the use of these sites is clearly becoming a productivity black hole. It is clear that businesses shouldn't turn a blind eye to their employees' use of social networks, and instead look to enforce usage policies."
The study found 76% of those surveyed had not been issued with specific guidelines relating to the use of Twitter.
"Without guidelines and usage policies businesses are leaving themselves wide open to a reduction in productivity, brand damage and security risks," said Mr Wicks.
The survey of 1,460 office workers was conducted by independent research company TNS.