The NHS, government departments, schools, councils and other public sector organisations will have to publish information on any gender pay gap from today, in a move described by unions as a "start" in tackling discrimination.
The new gender reporting legislation covers employers with at least 250 workers, extending to private firms from April 6.
A study by Totaljobs found that a third of employers were not prepared for the legislation.
A survey of 4,700 workers and 145 employers for the jobs site revealed that four out of five companies were not reviewing their equal pay policy.
TUC general secretary Frances O'Grady said: "Publishing information on gender pay gaps in salaries and bonuses is a start, but it is just that - a start.
"To genuinely tackle the root causes of the gender pay gap, we need to understand why women are still being paid less than men and do something meaningful about it.
"We need to remove the barriers that stop women going into better paid, male-dominated professions and create more better-paid part-time and flexible jobs.
"And we must improve pay for undervalued - but vital - jobs that are predominantly done by women, like social care.
"But this won't happen just with light-touch regulation. Government should extend the law to medium-size organisations as well as large employers, and bosses that don't comply with the law should be fined."
Sam Smethers, chief executive of the Fawcett Society, which campaigns on women's issues, said: "Employers should see this as an opportunity not a threat.
"Through gender pay gap reporting they can address the productivity gap and get the best person for the job at the right level.
"But we won't close the gender pay gap until we truly enable fathers to share the care of their children, make all jobs flexible working unless there is a good business reason not to, and address the harmful segregation in our labour market which sees women concentrated in low paid work while men dominate higher paid or more senior roles."
A Government spokesman said: "The UK is one of the first countries to require employers to publish their gender pay gap and bonus gap, covering approximately 9,000 employers and over 15 million employees – that's nearly half the national workforce.
"Reporting is the first step, allowing employers to identify the barriers to gender equality in their organisations and take action to make sure no one is held back because of their gender.
"This is backed up by our work to provide shared parental leave, new rights to flexible working and 30 hours free childcare to parents – all of which will help to tackle the gender pay gap."