Three quarters of UK employees claim not to access Facebook and MySpace during working hours, according to a survey by recruitment firm Badenoch & Clark.

This is despite the fact that Facebook, celebrating its fourth birthday today, now boasts more than 62 million active users and MySpace over 200 million.

The survey found that 38 per cent of UK office workers are not even signed up to a social network, while 22 per cent restrict their usage to out of office hours and a further 12 per cent have had access blocked by their employers.

Only six per cent of respondents feel that social networks are a major source of distraction to productivity at work, compared to instant messaging (eight per cent), pointless meetings (30 per cent) and loud distracting colleagues (52 per cent).

The survey also suggests that Londoners are the most prolific social networkers compared to any other region, with 38 per cent signed up versus the national average of 28 per cent.

"These days most of us have many elements to our online presence," said Andy Powell, director of marketing at Badenoch & Clark.

"But they do not always necessarily show us in the best light and could even land us in trouble when we're looking for that perfect next job."

The recruitment process has changed dramatically in recent years and more and more recruiters are taking note of "internet reputations", according to Powell.

However, there are legal implications around how recruiters and employers use information they find online, and legislation lays employers open to all sorts of claims of discrimination.

A 2007 survey by social networking site Viadeo found that one in five employers already use the internet to search for information on candidates.

Almost 60 per cent said that the information they found on the web would influence a recruitment decision.

"If employers are tempted to research their prospective applicants on social networking sites they must be careful not to use any information they discover to unlawfully discriminate against the applicant," said Sophie White, an employment solicitor at law firm CMS Cameron McKenna.

"However, apart from this, employers can use social networking sites as they please to learn about job applicants.

"Applicants cannot challenge adverse recruitment decisions based on the fact that the employer perceived that the individual was not serious enough."

Badenoch & Clark warned job seekers to create an online reputation that emphasises positive attributes.

"As a benchmark, never publicly post something you wouldn't want your mother to see," said Powell.