Changes to current law are needed if women's wage packets are to match those of their male colleagues, TUC deputy general secretary Frances O'Grady will tell a TUC equality law conference later today.
She will say that in the 35 years since the Equal Pay Act was passed, progress towards achieving equal pay has been painfully slow.
Present estimates show that female full-time employees now earn 19.8 per cent less than men working full time. O'Grady will call for changes in the law to allow unions to take group cases to tribunal.
"A tribunal hearing a case on behalf of 500 dinner ladies has to consider 500 separate applications," she said.
"It is not surprising that it can take years to win a case. Until unions are able to take group cases on behalf of female employees, the gender pay gap looks set to stay as wide as ever."
She will also call for new powers to force employers to show that they have carried out equal pay audits.
"Without more honesty about pay at work, employers will continue to hide behind the British obsession of not talking about salaries, and continue to pay their male and female employees, who do the same or similar jobs, differently," O'Grady said.