The Supreme Court's ruling means that 251 female classroom assistants, learning support assistants and nursery nurses can return to the Employment Tribunal to pursue an equal pay claim against their employer, Dumfries and Galloway Council.
The tribunal will now rule on whether the women's jobs were of equal value to those employed by the council on a full-time basis in a range of manual roles and, if so, whether there was a "material factor other than sex" why different pay arrangements applied to the two groups of workers.
Employment law expert Selwyn Blyth of Pinsent Masons, the law firm behind Out-Law.com, said that the case showed that UK courts were finally "getting to grips" with the complexities of equal pay law.
"We have had equal pay legislation in this country for nearly 40 years, but for a long time employers have been able to avoid arguing the substantive issues of why certain groups of workers have been paid differently to others using jurisdictional arguments, such as whether roles are of equal value or whether claims are out of time," he said.
"In recent years there have been a number of cases in which the courts have refused to allow these jurisdictional arguments to distract from the purpose of equal pay law: to ensure that workers receive the same pay for work of equal value," he said.
Although many of the cases currently in front of the courts had been brought under the previous Equal Pay Act, Blyth said that the same rules would apply to post-2010 Equality Act cases. These cases of "occupational segregation", in which certain types of jobs tended to be performed by those of one gender, were unlikely to be deliberately discriminatory, he said. However, they can give rise to a special form of indirect discrimination where men or women gravitate towards certain types of job assessed as being of equal value but paid differently.
Under the Equal Pay Act, men and women should be treated as being in "the same employment" if they are "employed by [the] employer or any associated employer at the same establishment" or "at establishments in Great Britain which include that one and at which common terms and conditions of employment are observed either generally or for employees of the relevant classes".
The female employees worked in various schools in the local authority during term time, and were employed under conditions set out in a national collective agreement known as the 'Blue Book'. The male workers they proposed to compare themselves to were employed full time, under conditions set out in the 'Green Book' collective agreement. This included an entitlement to a substantial supplement on top of their basic pay.
In her leading judgment, Lady Hale said that it was irrelevant that there was no "real possibility" that the manual workers would ever be employed in the same schools as the women bringing the claim. The correct test was whether, if they were to be transferred to a different location to do the same job, the manual workers would retain the same employment conditions, she said.
"This is not just a matter of preventing employers from so organising their workplaces that the women work in one place and the men in another," said Lady Hale. "There may be perfectly good reasons for organising the work into different places. But the object of the legislation is to secure equality of treatment, not only for the same work, but also for work rated as equivalent or assessed by the experts to be of equal value."
"It stands to reason, therefore, that some very different jobs which are not or cannot be carried out in the same workplaces may nevertheless be rated as equivalent or assessed as having equal value ... The fact that of necessity their work has to be carried on in different places is no barrier to equalising the terms on which it is done," she said.