For anyone with a case of mild Facebook addiction, finding the time to squeeze a little work in between messing around online has become one of the great challenges of the 21st-century office. So if you are taking a quiet moment at work to read this online, steal a glance over your shoulder now: an investigation by the Guardian has found that employers are taking an increasingly draconian line on workplace time-wasters.

More than 1,700 public employees have been sacked or disciplined for internet or email misuse in the past three years, our research has found.

The figures - obtained from 65 institutions - show how strongly employers are clamping down on staff who spend hours on social networking sites such as Facebook, MySpace and Bebo.

Unions say that disputes over the sites are growing at a phenomenal rate and have demanded clearer guidelines for their use. Studies have shown that up to £130m a day in productivity is lost because of the sites, with Facebook's British members spending an average of 143 minutes a month logged in.

Other common offences include excessive use of other websites or email, viewing "inappropriate" material such as pornography and the forwarding of offensive emails. An increasing number of employers now attempt to ban sites they consider inappropriate. Government departments have been among the most unforgiving. The Ministry of Justice (MoJ), formally the Department for Constitutional Affairs, dismissed 30 employees while the Department for Work and Pensions reprimanded 313.

An MoJ spokesman said that the offenders had been in clear breach of the department's IT procedures and, in some cases, its equality and diversity rules. Personal internet use is only allowed if sanctioned by a line manager.

The Metropolitan police disciplined 187 staff while Hertfordshire police gave formal warnings to 25 after an offensive video clip was circulated. The TUC has criticised employers who clamp down too heavily on staff without properly fleshing out guidelines for what constitutes reasonable conduct. A spokesman said: "Social networking at work is a recent problem and it's growing at a phenomenal rate. But employers are often not setting a standard as to what level of internet use is acceptable."

The figures, obtained under the Freedom of Information Act, show that in the past three years at least 1,722 public employees were disciplined for internet or email misuse. A total of 132 were sacked, 41 resigned, 868 got formal warnings and 686 received other forms of warning or punishment, such as a reduction in rank or a fine. The data came from 65 government departments, police forces and local councils.

A spokesman for the Local Government Association said: "Councils will take a no-nonsense approach to anyone who uses the internet in work time to download inappropriate or offensive material. Councils have a duty to ensure that employees are working for the council taxpayer as that is who pays their wages."

Cary Cooper, a professor of organisational psychology and health at Lancaster University, said that managers should be realistic. "Britain has some of the longest working hours in the developed world. Employers have created this culture. It is natural for people to have to use work computers for organising their personal life." Social networking has boomed over the past 12 months. Facebook alone now has more than 5 million regular users in the UK.

A spokesman said: "While we respect the decision of employers to put website policies into place, we also encourage them to consider the important role Facebook plays in people's daily lives."

Caught in the act

· Metropolitan police officers in Lewisham were in hot water last summer after posting a bizarre series of video clips on Facebook. One revealed that he loved his job because "I get to hit people with a stick."

· Nine workers were sacked by Neath Port Talbot council this year after spending up to two hours a day on eBay.

· 140 officers and staff of Hertfordshire constabulary were caught up in an investigation last year after a video from the United States showing a man being decapitated was circulated.

· Former IBM employee James Pacenza, 58, is suing the firm for £2.5m in the US after he was fired for visiting an adult chatroom - claiming he is a chatroom addict because of stress arising from seeing his best friend killed in the Vietnam war.