A further 14% do not know whether their company has these policies in place.
The growing numbers of employers said to be concerned with the impact of social media on their organisations prompted the research by Croner, part of Wolters Kluwer.
As well as the 15% looking at the internet once an hour, the online survey found 6% of workers are doing this more than three times an hour. Male employees are the biggest offenders with 19% confessing to logging on at least once per hour compared to 10% of females.
And it is bad news for the capital's businesses as Londoners are the most likely to surf the net for personal use during the day (42%).
Amy Paxton, senior employment consultant at Croner, said: "The numbers of bosses calling our employment advice lines about social media use in the workplace has steadily risen noticeably over the last 18 months.
"Traditionally employers have had a knee-jerk reaction to social media, some wanting to dismiss employees for gross misconduct. However, this could result in claims for unfair dismissal.
"Additionally, when faced with such situations, employers may also want to introduce a complete ban on accessing social media sites. While this is an understandable approach, employers need to consider the potential benefits for their business if employees make positive use of social media.
"It is completely acceptable for employers to limit internet use during times when employees are expected to be working as it obviously has an impact on productivity. However, access could be offered before or after work, or during official breaks. Whatever an employer chooses it is important that they have the right policies in place which clearly set out what employees can and can't do.
"High-profile legal cases involving Twitter and Facebook users at work should serve as a warning to employers of the dangers of not having clear policies in place.
"Even though social media still presents itself as a challenge to many businesses it doesn't have to. And if companies want to be seen as innovative, exciting and dynamic then they may wish to hop on the social media bandwagon and start to future-proof their business."