Mrs Wilson was employed as a band 3 inspector by the Health and Safety Executive (HSE). In July 2002, she issued an equal pay claim. Wilson relied on three comparators who were paid more than her, each of whom was also employed as a band 3 inspector. In part, this was because of their longer service. Broadly, the HSE's pay scheme provided for pay to increase with length of service, up to a maximum of 10 years. Wilson argued that the HSE's pay scheme was not justified.
In the case of Danfoss, the European Court of Justice (ECJ) had ruled that pay differences arising from service criteria do not have to be justified on the basis that rewarding experience is a legitimate objective. This was qualified in the later case of Cadman, which held that employers have to show objective justification where a claimant provides "evidence capable of raising serious doubts" as to whether that criterion is appropriate "as regards a particular job".
The employment tribunal first heard the case in 2003. At this stage, Cadman had not reached the ECJ. Therefore, applying Danfoss, the tribunal held that Wilson's claim had to fail.
Wilson appealed to the Employment Appeal Tribunal (EAT). The appeal was stayed while Cadman went before the ECJ. Then, in light of the Cadman decision, the EAT remitted the case back to the tribunal to determine whether Wilson had provided evidence capable of raising serious doubts about the appropriateness of using a length of service criterion to determine pay in her job. The tribunal held that she had not, and that once it was accepted that the criterion had been appropriately applied, the tribunal could not question the way in which it had been applied.
Wilson appealed to the EAT again, arguing that the tribunal's scrutiny should not end with asking whether a length of service criterion is appropriate it should