People working voluntary overtime could claim for additional holiday pay. Currently, only basic pay counts when calculating holiday pay.
The details of the ruling, particularly on whether claims can be backdated, have yet to be released.
The ruling could be appealed, meaning a final decision may be years away.
The ruling has widespread implications for all companies paying overtime to their staff.
The government estimates that one-sixth of the 30.8 million people in work get paid overtime. This means around five million workers could be entitled to more holiday pay.