Fees for those bringing employment tribunal claims have been ruled unlawful, and the government will now have to repay up to £32m to claimants.
The government introduced fees of up to £1,200 in 2013, which it said would cut the number of malicious and weak cases.
Government statistics showed 79% fewer cases were brought over three years - trade union Unison said the fees prevented workers accessing justice.
The government said it would take steps to stop charging and refund payments.
The Supreme Court ruled the government was acting unlawfully and unconstitutionally when it introduced the fees.
Unison general secretary Dave Prentis said: "The government has been acting unlawfully, and has been proved wrong - not just on simple economics, but on constitutional law and basic fairness too."
He added: "These unfair fees have let law-breaking bosses off the hook these past four years, and left badly treated staff with no choice but to put up or shut up.
"We'll never know how many people missed out because they couldn't afford the expense of fees."
