Hands up if you have ever told a lie on behalf of a colleague or superior at work. Honestly? Not even a little white one? Most of us will have at some time or other. But the challenge is knowing where to draw the line. According to Christine Allan, the personal assistant to an oil company boss who was on trial for fraud last month, lying is standard practice in City life. She was found to have faked documents to smooth the passage of a pounds 7.2 million City deal. In her defence she denied any wrongdoing, suggesting that she was, in part, attempting to protect her boss.

Such lies, of course, come in various sizes. Taking a call for someone and telling the caller that person's out when actually, they don't want to take the call, must surely be a daily occurrence in many offices. But for some, like Ms Allan, bigger lies can have far-reaching repercussions.

"Most of the problems we have come across relate to senior members of a team asking a younger team member to cover up a situation for them," says Peter Norbury, employment partner at law firm Eversheds.

"These can be extremely difficult to unravel because you only look at the situation when it comes to light, retrospectively. Often, when you do, it turns out that a number of people may have known about some sort of deceit, but having been sworn to secrecy, they then perpetuate it, and become part of it."

A classic area where such problems arise is accountancy, he claims. Often, stock is valued at a greater level than its true worth. Another typical scenario involves income for one year being put down as income for another.

The exploitation of a close professional relationship between an assistant and his or her boss is even more serious. In court last week Mohamed Al Fayed's former secretary was accused of lying on behalf of her boss about cash payments to Neil Hamilton. The suggestion made by Mr Hamilton's lawyers was that she had done so out of misplaced loyalty to the Harrods boss - a claim she strenuously denied.

"In some strong relationships the boss relies on the assistant to maintain and conceal deceit," explains Mr Norbury. "In that situation, however, the assistant is particularly vulnerable." Indeed, so long as your boss is safe, you are too, but should anything happen to them, your loyalty to that individual will be deemed by his or her superiors as disloyalty to the company you work for.

In such cases, underlings complicit in some sort of deceit at work are most likely to be asked to leave quietly. As few have the inclination to defend their lies, only a small number of such dismissals would ever result in a tribunal. The most complicated aspect of the issue of lying at work is that different individuals have different moral codes. While most of us know the difference between right and wrong, the definition of what is a "white lie" - in other words, an acceptable deceit - will vary, says Paul Jacobs, director of corporate affairs at Office Angels.

Research by the company, published in November, revealed that the question: "Would you ever lie in the interests of your job?" is among the five "killer" questions some employers now use in job interviews.

"While I would advise anyone to be completely truthful about their skills and qualifications when applying for a job, if asked this you'd have to say 'no'," Mr Jacobs admits, although he says the reason the question is asked is that you're damned if you do, and damned if you don't.

Once in a job, you must weigh up the severity of the lie and the scale of possible implications from it, Mr Norbury says. "There is whistleblowing protection for people in a serious situation. But lies with less serious consequences can get you into uncertain waters," he explains.

"Lying to keep a problem at bay until a time when you can deal with it properly is one thing," Mr Norbury adds. "But the problems start when such a lie has a negative consequence, at which point it becomes impossible to defend."

After all, the minute you've told the lie you're on the back foot, he says - a very different situation to having told someone something that turned out to be a lie but you believed to be true.

In any business, there are people who regard integrity as absolute, and other people who don't, says Mr Jacobs. It's all down to personal morality, and the best defence is to decide what you would and wouldn't do up-front, then apply this with a dose of good sense to any unforeseen circumstances you encounter.

Mr Jacobs concludes: "In my view, your first obligation is to yourself. If you consider something morally abhorrent or illegal, then run in the opposite direction as fast as you can."