The move marks a step forward in Bernadette Cadman’s fight against her employer, the Health and Safety Executive, after she discovered she was paid less than the average salary of male colleagues on the same grade.

Prospect, the union backing Cadman’s claim, described the date for the start of the hearing as auspicious, given the disproportionate impact that seniority-based pay systems have on women.

Cadman worked for HSE for 13 years and was promoted to a band 2 inspector in 1996. After checking with personnel she discovered male colleagues were being paid in the order of £5,000 to £7,000 more per year.

Prospect General Secretary Paul Noon said: "While the case is particularly relevant to public services it will also be significant in any employment where long seniority-based pay scales exist, or where additional contractual benefits are dependent on long service such as enhanced holiday entitlements.

"When service-related pay is analysed women are often clustered at the lower parts of the pay band. This is because statistically women on average have shorter service, often due to children or other care responsibilities. The increase of women entrants to traditionally male dominated professions also raises questions over seniority-based pay."

Mrs Cadman's solicitor, Emma Hawksworth, a partner at Russell Jones & Walker, said: "This is the most important equal pay claim to be brought in the last 10 years.

"The equal pay act makes pay differences between men and women doing the same job unlawful, unless the pay differential can be shown to be a result of a material difference between their cases. That material difference cannot itself be discriminatory, either directly or indirectly.

"The Equal Opportunities Commission has shown that in this country, and throughout the EU, the length of service of female workers, taken as a whole, is less than that of male workers, due in large part to female workers' domestic circumstances and obligations.

"In Mrs Cadman's case, we argued that it was unlawful to allow for men in a comparable role to be paid more than her solely on the basis of length of service and without any need to show why that was justifiable. "

Prospect will support Cadman’s case to its final conclusion. It is encouraged by indications in the UK Court of Appeal's decision that length of service pay schemes are not always self-evidently justifiable, and by the European Commission's written observations which support the position that seniority based schemes do require justification.

In May 2002, backed by the union, Cadman took her case to an Employment Tribunal, which found that women were disproportionally adversely affected by pay systems based on length of service. The tribunal found that HSE could not objectively justify the differences in pay.

But an Employment Appeal Tribunal in July 2003, citing an earlier European Court of Justice decision, ruled that HSE was not required to produce specific justification. Despite this setback the union, and their solicitors Russell Jones & Walker, pursued the case through the Court of Appeal, which expressed sympathy with Prospect’s argument and referred the case to the ECJ for today’s hearing.