A Court of Appeal ruling that a music teacher's holiday pay was wrongly calculated has been hailed as a victory for part-time workers.
Judges ruled in favour of Lesley Brazel, a music teacher who works about 10 to 15 hours a week at Bedford Girls' School during term-time.
Unison said the ruling affects "hundreds of thousands of employees working part-time and irregular hours".
The union said it would have an "impact across the whole of the UK".
Holiday pay for part-time workers should be based on average weekly pay, the judges said.
The school had allocated Ms Brazel's holiday pay as a percentage of yearly hours worked, while the judges said it should be based on her average weekly pay over a 12-week term.
'Valued member of staff'
Unison intervened after the Harpur Trust, which runs the school, challenged an earlier decision by a tribunal judge who concluded that Ms Brazel's holiday pay had been wrongly calculated.
That decision was upheld on Tuesday by three judges, who said the method of "pro-rating" used by the trust to work out holiday pay - which resulted in a lower figure - was not provided for within the working time regulations.
In a statement after the judgment, Unison general secretary Dave Prentis said: "Unison intervened because this case was about all workers being treated fairly and would have an impact across the whole of the UK."
Ms Brazel's claim was originally dismissed by an employment tribunal in Bury St Edmunds in January 2017.
But she won an appeal in March last year, which meant she was entitled to receive holiday pay at the higher rate she had contended for.
The trust's lawyers said during the Court of Appeal hearing in May that Ms Brazel continues to teach at the school and is a "valued member of staff".