The final stage of a campaign against controversial fees to take a case to an employment tribunal goes to the Supreme Court on Monday.
Unison has been taking legal action since the introduction in 2013 of charges ranging from £390 to £1,200.
A two day hearing in the Supreme Court is the final stage of Unison's legal campaign.
A government review of the impact of fees showed a 70% drop in the number of cases since July 2013.
Low-paid women, especially those treated unfairly when they were pregnant or on maternity leave, have been the biggest losers, said Unison.
The union's case to the Supreme Court will assert that the fees have stopped many thousands of badly treated employees, especially those on low incomes, from getting justice.
General secretary Dave Prentis said: "If an employer breaks the law and treats one of their employees unfairly, they should be challenged. It cannot be right that unscrupulous bosses are escaping punishment because people simply don't have the money to pursue a case
"The introduction of fees was a terrible decision. It has denied many thousands of people the right to seek justice. Bad employers are having a field day, safe in the knowledge that few will be able to afford to challenge them at a tribunal.
"The G overnment originally said making people pay would weed out vexatious claims. All it's done is penalise lower-paid employees with genuine grievances. That's why it's so important our legal challenge succeeds."