Companies will be forced to reveal how much they pay male employees compared to the salaries women receive, under legislation designed to tackle the gender pay gap to be published next week.

All companies employing more than 250 people will be obliged to publish a percentage figure showing how much more men are paid than their female colleagues.

The equalities minister, Harriet Harman, has said that the first step to tackling pay inequalities is to increase transparency, so that women know whether they are being paid less then men. On average, women working full-time receive 17% less than their male colleagues, and female part-time workers are paid 36% less.

The measure will not require companies to conduct comprehensive pay audits, based on evaluating the individual salary of every staff member, but will be much simpler to calculate – by averaging men's hourly salaries and those of women, and making a company-wide comparison.

Details of how the figures should be compiled and published will be worked out by the Equalities and Human Rights Commission (EHRC) later in the year.

In the light of recent research by the EHRC showing that men's bonuses and annual incentive payments were on average 79% higher than those of women, companies will be obliged to include bonuses in the calculation.

Public sector organisations with over 150 employees will also be required to publish a similar statistic.

Companies that have a large gender pay divide will then be given until 2013 to improve their record. If they take no action, they could face prosecution. The full details of the measure will be included in the equalities bill on Monday. In the interests of greater transparency, the bill will also ban "gagging clauses" that prevent individuals from disclosing their salaries.

However, equal pay campaigners warn that this will have little impact because not many companies have gagging clauses and, in any case, people in Britain tend to be reticent about discussing their pay.

There was concern from the business community about the cost involved in compiling these statistics, and the cost of paying for retrospective equal pay claims that could be triggered by the publication of the figures.

But the union response was more positive. Bronwyn McKenna, Unison's director of organising and membership, said she would welcome any measure that helped to tackle the pay gap. "Too few women currently get equal pay through the courts. It is a lengthy and hugely costly process which forces women to jump over numerous technical hurdles," she said.