A former staff member of Safeway supermarkets sent a rogue email to UK customers of the chain informing them that Safeway would be increasing its prices by 25%. And if they didn't like it, they could shop elsewhere. The case, which took place in 2000, was to be one of the first major incidents of cyber-crime, which has since spread like wildfire through British offices, with data theft, intentional computer damage, viruses and email infringements all becoming major problems for employers.

So why do we so rarely hear about examples? According to the FBI, cases in the US - which are even more prevalent than here - are "phenomenally unreported" by companies because they don't want to admit that their security policies are flawed. The bureau's seventh annual computer crime and security survey, published last week, shows that 90% of American respondents detected a security breach within the past 12 months. However, only 34% reported the intrusions.

Sometimes the culprits are anarchists and hackers. "Unused IDs, passwords and remote access permissions are a magnet for them, even the amateurs," says Chris Christiansen, analyst for International Data Corporation. Take the annual anti-capitalist protests that take place in May. They have, in the past, involved police in the City of London issuing a warning to companies in the country's financial heart to be on their guard.

Alternatively, perpetrators can be fraudsters and thieves. These people may steal huge amounts of money and then destroy the evidence by vandalising the computer system. They often pose as employees - usually in secretarial posts rather than IT roles because they cause less suspicion and involve a higher turnover of staff. Where theft is of data that might be valuable to, say, a competitor, the culprits are more likely to be disgruntled employees or former employees.

In fact, the vast majority of cyber-saboteurs, ranging from people creating viruses to sending rogue emails, tend to fall into this latter category. A study by the American Society of Industrial Security found that vengeful employees are now the biggest security worry for 90% of US bosses. And while less research has been carried out in the UK, anecdotal evidence confirms that we are catching up fast.

Corinne Usher, a consultant clinical psychologist in Amersham, Buckinghamshire says: "Employees who have been moved from long-term to short-term contracts, employees who have been demoted or sacked, or employees who are generally devalued, often feel hurt and want to get their own back. They choose to do so through computer systems for several reasons. The main one is that you can do a great deal of damage without even leaving your chair."

It is significant that what happens to people at work tends to be quite public, says Usher. "It means that the internet is an excellent medium by which they can get revenge because it is so public. In other words, people feel they can do to someone else exactly what has been done to them."

In large corporations, where cyber-crime is most common, it is never one single person who makes the decision to, say, demote you, she adds. "It makes it hard to point a finger of blame at an individual because it is the whole system that's responsible. Viruses tend to be a particularly attractive way of seeking revenge in these cases because they offer a rare chance to attack the whole system."

Mark Griffiths is a reader in psychology at Nottingham Trent University. He has found that people are uncharacteristically uninhibited when using the internet. "My research on online relationships reveals that people fall in love online a lot quicker than in the outside world. They feel they can let their guard down and allow emotions to flow," he says. "Internet revenge is an example of the other end of the emotional spectrum - hatred rather than love."

And because you only have to hit a few buttons, you can kid yourself that you haven't really done anything wrong. All the research shows it is easy to dissociate yourself from actions. "In fact, a huge number of people think that they're protected by anonymity. But wherever you go on the internet, you leave electronic footprints. Every time you visit a site, you leave an electronic calling card called a 'cookie'. People usually think that if they've downloaded porn but deleted their files they won't be found out. But they can always be found out, and easily," says Dr Griffiths.

It is not just employers who are being targeted. Employees, too, are increasingly at risk from cyber-sabotage. Many have, for instance, come into work and found that their email inbox has received 300 junk-mail messages overnight. "We recently dealt with one case of an employee receiving a bounced message that he never sent. That message was pornographic," says Kevin Townsend, founder of www.itsecurity.com. "Since downloading porn is a sackable offence, the consequences could have been very serious for him."

A particularly calculating colleague might opt for sending you an advanced virus, which anti-virus company Sophos claims are becoming more and more common. "Most of us think of viruses as the things written by random youngsters who think it's cool to wipe as much as possible in as short a space of time as possible. The real problem occurs with the recent increase in the more sinister, unrecognisable viruses that have more subtle corruptions. They might, for example, multiply a random number on your spreadsheet every now and then by 0.05, or wipe random paragraphs from text files over several months," says a spokesman.

These sophisticated viruses, he says, are considerably more dangerous. "First, they're less easy for anti- virus software to spot, and second, by the time you've noticed the effects, all your back-ups will probably be corrupted, too."

Today's culture of presenteeism, coupled with more people being packed into smaller offices, can be to blame for attacks on colleagues, says Marilyn Davidson, an organisational psychologist. "Petty irritations have long been a source of warfare in the office environment. But modern pressures at work - including longer working hours and smaller spaces in which to work - mean that they are more likely than ever to affect staff relations and blow up into something bigger."

A recent report by human resources consultancy Eden Brown found that angry employees are fast on the increase and that they are taking it out on their colleagues. Its survey of 2,000 UK employees found that the office rage hotspot is greater London. "This is probably because London has the most competitive and pressurised workplaces," says Ian Wolter, managing director.

An early warning sign that you may be in the presence of a potential cyber-criminal is the sending of "flame-mails". To the unacquainted, "flaming" is defined by someone losing their self-control and sending emails that are derogatory, obscene or inappropriate. A few years ago, Susan Ellwood (not her real name), a 33-year-old PA from London, began to be regularly faced with messages from her manager such as: "Kettle not working today, love?" or "Spelling errors in the report again. Is there a brain lurking inside that head of yours?" Last year, this same manager was caught trying to send viruses in an attempt to get back at a workplace he increasingly despised.

Not surprisingly, the sudden rise in cyber-spite has caused many companies to take action. CCTV is becoming a normal part of working life in some offices and heavily restricted access and back-up systems are also becoming common. You may find that you're subjected to "cyber-snooping" - employers monitoring your email.

Jeff Drake is executive vice president of corporate planning and strategy at Access360, which advises employers on IT security. He claims that smart companies are acting, not just reacting. Now the priority for employers is not catching cyber-criminals in or after the act, but preventing them from acting at all.

If all else fails, some companies call on "intelligence operatives" such as Anthony Benson. Phone-tapping, hidden cameras, computer keyboards that monitor all activity of the person operating them, are all part of his private detective role. "Often, I pretend to be a new employee," he says. "I befriend the suspect and find out if they're up to anything. If they are, I get proof and the employers take it from there."

You have been warned.