The PSNI and the Police Authority face a holiday pay bill of up to £30m after a tribunal ruled in favour of a class action brought by a group representing more than 3,700 officers and staff.
A judge said staff were owed money as their holiday pay had been based on basic working hours, not the actual hours worked including overtime.
He said the pay issue was "extremely serious" and breached European law.
The PSNI has been contacted for comment by BBC News NI.
The case was brought by a group of 14 lead claimants, 11 police officers and three civilian staff members.
The group represents police constables and sergeants, to whom Chief Constable George Hamilton is answerable - and civilian support workers, to whom the Northern Ireland Policing Board is answerable.
The industrial tribunal's decision could result in the two bodies facing an estimated bill of £30m in relation to the unlawful deductions dating back 20 years.
Solicitor John McShane, from McCartan Turkington Breen, represented 11 of the claimants.
"The issue in question was whether these groups should have their overtime and other allowances included in the calculation of their normal pay, in order to work out their holiday pay entitlements," he said.
"Up until now, holiday pay has been based on working basic contracted hours and has not taken into account the often extensive additional hours required by their work."
He said the legal claim was based on a case in Britain in 2014 in which the Employment Appeal Tribunal ruled that overtime should be included in holiday pay.
Legislation was introduced in England and Wales to limit backdated claims to two years, but Mr McShane said no equivalent legislation was passed by the Northern Ireland Assembly.
Delivering the industrial tribunal's decision, Judge Kelly said that overtime is often a requirement of working for the police service.
But he said that the failure to provide the correct annual leave "is an extremely serious matter going to the heart of one of the fundamental rights of the European Union".
In reaching his decision he said: "The present claims appear to highlight a situation where a clear entitlement under the Working Time Directive has been effectively ignored and avoided for a significant period of time and continues to be avoided.
"It is particularly concerning that this has been done by a public authority."
Mr McShane said the chief constable had previously written to claimants, proposing that in future staff would receive holiday pay, including overtime, averaged out over the year.
"This offer was too little too late by the chief constable and we are therefore delighted on behalf of our clients that the tribunal has expressly stated that that there is no limit on the amount of deductions which can be ordered to be paid by the employer," said Mr McShane.
"We will now be entering into negotiations to bring a financial conclusion to the matter."