More than two thirds of London companies - led by top City firms - are now banning or restricting the use of internet site Facebook over fears that staff are wasting time on it.

Many firms have now warned employees that "Facebooking" during office hours is a sackable offence.

An Evening Standard straw poll of major employers found that more than 70 per cent of the businesses, including banks, law firms, utility companies and government departments, have banned or limited access to social networking sites. Recent figures showed London has overtaken Toronto as the city with the most Facebook users in the world.

More than 826,000 people are registered on the site, a figure that has doubled since May.

The site was first banned by several American and Canadian companies who noticed the large amounts of time employees were spending on it.

A study found British users spend on average 191 minutes a month on Facebook and dozens of people have admitted to "Facebook addiction", where they check on their friends, and often exes, compulsively.

Our study found British Gas, the Met, Lloyds TSB and Bristows law firm all had internet filters preventing sites such as

Facebook, MySpace, Bebo and Hotmail being viewed at work.

A spokesman for Credit Suisse said: "Staff are forbidden from accessing the site while at work as it is thought that they are wasting company time and money."

A member of staff at investor Dresdner Kleinwort said: "The ban is widespread across all banking offices."

A spokesman for the law consultant-firm ELAS said companies-were well within their rights to sack staff for logging on to Facebook and that the site had caused numerous problems.

"Most contracts nowadays have a clause which restricts internet use to business-use only," he said.

"A manager could quite easily sack someone if they caught them using these social networking sites during office hours. Unless you can argue it is an important work tool, people should not be accessing these sites during work time."

Some companies have imposed "Facebook time" for employees.

"We have imposed a partial ban, allowing employees to access it during their lunch hour," said Amanda Turner of IT recruitment company MDA resources.

"We did have a problem with people using it for far longer than they should."

The internet site, which now has four million users in Britain, was set up in 2004 by 23-year-old American student Mark Zuckerberg.

He is facing a legal threat from a rival website, ConnectU, the owners of which allege they and Zuckerberg conceived Facebook while at Harvard University.

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