Teachers and school staff on zero-hour contracts who are not paid outside term time are set to benefit from a Court of Appeal ruling about holiday pay.
Leading judges ruled in favour of Lesley Brazel, a clarinet and saxophone teacher who works about 10 to 15 hours a week at Bedford Girls' School during term time.
The court judgement, published today, describes her as a “part-year worker”, who has a continuing employment contract but whose hours differ from term to term and between term time and holidays.
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Public sector union Unison intervened in the case 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 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.
The trust today said it may appeal to the Supreme Court.
The union said the ruling affects "hundreds of thousands of employees working part-time and irregular hours or patterns".
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.
"Staff in schools are often required under contract to be at school outside term time and, like Ms Brazel, are required to do additional unpaid duties beyond those periods.
"It's right that they should be paid the same minimum statutory entitlement of 28 days' annual leave like everyone else."
Unison legal officer Shantha David, who acted for the union in the case, said: "We're extremely pleased that the Court of Appeal considered and accepted Unison's argument.
"The government's failure to provide guidance in this area has left workers in limbo.
"The courts have once again had to step in to stop the abuse of workers and to fix what legislation should have made clear from the outset."
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.
Upholding that decision, Lord Justice Underhill said the method of calculation required by the working time regulations is "straightforward and should be followed".
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".
A spokesperson for the Harpur Trust said: "The practice of pro-rating holiday entitlement for staff who only work for part of the year was widely understood to be correct and, in line with the approach recommended by the government's Advisory, Conciliation and Arbitration Service (ACAS) in their guidance, and it has been applied nationally in many industries and sectors.
"As has been acknowledged throughout this case the application of the relevant legislation to employees who work part of the year has been unclear.
"Given the highly technical nature of this issue it has had to be determined by the courts and further appeal may be made to the Supreme Court.
"The Harpur Trust values all its staff and will ensure that they receive pay and benefits in line with any changes to legal requirements when this issue is finally determined."