Rebecca Gill, of the Trades Union Congress (TUC), says employers are to blame for not treating staff equally. But she adds: "An additional problem is that women often find it harder to ask for a fair deal on pay - anecdotal evidence suggests men are much more prepared to take risks in the workplace."
It's not just women who struggle. Florence Kennedy, managing director of Negotiate, an Edinburgh-based company that trains people to ask for a better deal, says: "Both men and women can get embarrassed about asking for more money but, even worse, when they do ask, they often don't do it in the right way."
The two things most likely to damage your case for an increase are lack of evidence and aggression. A request for a rise simply on the grounds that you think you're worth more is likely to be rejected. Similarly, tantrums rarely pay dividends - losing your temper is likely just to antagonise your boss.
Your starting point should be to find out as much as possible about the market value of your job. Colleagues may be reluctant to discuss the precise details of what they earn, but it may be possible to find out about the range of salaries your employer pays to people at your level. If you are a member of a trade union, it may have data on what colleagues are paid.
It is also possible to find out what people doing similar jobs at other employers earn. "Our online salary checker will give you a much better idea of what someone doing a similar job to you is getting," says John Wood, of PayWizard.co.uk, an internet site set up in partnership by the TUC and Incomes Data Services.
Establishing that you are underpaid relative to colleagues or staff at rival companies is a good start. But it will be even easier to fight your case if you can demonstrate that you deserve rewarding on your own merits. Check your job description and consider how you perform against it. Employers are more likely to offer rises to staff who do everything their job descriptions require.
It may be possible to argue for a raise on the basis of a specific contribution to your company. This is easiest in jobs where people are responsible for bringing in new income or reducing costs. Think about projects you have managed or led, or awards you have received. Are you performing roles not originally in your job description? Do you have any written testimonials from clients or colleagues?
Negotiate suggests there are four key phases to any negotiation exercise. Preparation - building your case for a pay rise - is the first and most important of these phases, but the next three - debating, proposing and bargaining - are crucial too.
Kennedy says: "Try to behave in a way that you would like to be treated yourself. Listen to what your boss has to say and ask open questions to encourage a rapport - it is usually counter-productive to be aggressive, defensive or rude."
If you think you'll get nervous during the negotiation, don't be afraid to arrive with notes. You are also entitled to bring someone with you to discussions with your boss. This may be a trade union rep, or a colleague.
Having discussed the evidence with your boss reasonably, move on to the proposal stage. To do this, you need to have spent some time thinking about how large a pay rise to request. Be realistic about how much you're owed and what the company will be willing and able to pay. Equally, don't be a pushover. "Don't be embarrassed to aim high," says John Wood.
But be reasonable. You may have to accept a certain amount now, to be followed by a further rise later. You may have to agree to hit targets. or be prepared to compromise by accepting better fringe benefits rather than more money.
The final step in the negotiation is bargaining - closing the deal. Make sure you understand exactly what you have been offered - get it in writing to prevent future disputes. Agree timescales in which your demands - and any concessions, such as extra duties - will be implemented.
Behavioural psychologists encourage negotiators to use positive, open language to keep the atmosphere constrictive. Subjunctive expressions - "I'm sure you would agree" - are particularly effective.
Watch out for your employer's own techniques. One common ploy the "escalating authority" trick, where you are asked to negotiate with someone who does not have the power to sign off your pay rise. To avoid this, ask to speak directly with the manager who will make the decision over your salary.
Not everyone will succeed in getting what they want. In which case, plan your next step. One option is to ask your employer to agree a date in the future when it will reconsider your request. Or you can ask for new performance targets, where success will trigger a pay rise.
Even if your employer saying no is the final straw at work, don't threaten to hand in your notice - unless you have a job to go to.
Equal pay for equal work
* If you're requesting a salary increase because you are being paid less for reasons of discrimination, you have basic legal rights. "People have the right to be paid equally not just for the same job but also for jobs of equal value," says the TUC's Rebecca Gill.
* Two supermarket cashiers, say, should be paid the same rate. But they might also argue for the same pay as warehouse staff, on the grounds that they make similar contributions to the company.
* Typically, it's women who are underpaid, but equality laws apply to all staff. If you think you're being paid less because of your gender, talk to your employer. Find out why it pays you what it does.
* To successfully argue a case, you need evidence that your colleagues are earning more. Staff are often shy about discussing salaries. However, since 2003, employees have been entitled to ask their bosses to fill in an equal pay questionnaire, giving details of its pay grades and salary ranges.
* Asking your employer to fill in the questionnaire is the first step in appealing for help to an employment tribunal. Cases have to be fought individually.
* Your trade union will be able to help - and to represent you in hearings with your employer.
* If you're not a union member, ask for advice from your local Citizens Advice Bureau, or from the arbitration service Acas (08457 474747, www.acas.org.uk).
'I showed I had beaten my targets'
Paul Sumpter, 27, was convinced that he was being underpaid by his employer, a chain of fitness clubs, but his boss initially rejected the requests that he made for more money.
"I worked in sales and I quickly realised that the only way that I would be able to get more money was to make a financial case for an increase," says Paul, who is from Cambridge. "I went back through my sales figures for the year and I put together a document that showed how I had beaten all of my targets."
Paul found that his case was boosted considerably when he was offered another job working for a rival fitness club.
"I didn't want to leave," he says, "but this gave me the opportunity to find out what people who were doing my job at a rival company were earning - it was significantly more."
After compiling a dossier to support his case, Paul went back to his manager and made a formal request for a salary increase to be considered.
"I didn't threaten him at all," he says, "but I did point out that other companies would pay me more and that I was worth keeping, given my sales record." After negotiations had taken place, Paul's boss gave in and agreed to pay him £2,000 a year extra. Paul was also promised that he would be given a promotion if he continued to exceed his targets.
In the end, however, Paul became frustrated with the job he was doing and left. He is now working as a bank manager.