The landmark ruling, described as the most important sex discrimination judgment for ten years, means that companies can legally pay some workers more for length of service even though a woman’s ability to compete on time served will be curtailed by her decision to have children.

Bernadette Cadman, 44, a health inspector from Manchester, brought the case after she realised that she was being paid up to £13,000 less a year than male colleagues who were doing the same job.

Ms Cadman won her case at an employment tribunal in Britain but an appeals court referred it to the European Court of Justice for a ruling on the principle because of doubts over the implications of the ruling.

Last night her union Prospect, which supported her action, tried to put a brave face on the judgement by pointing out that employers could no longer use length of service to justify pay differences “unreasonably”. But the Luxembourg-based court’s ruling clearly shifts the burden of proving a claim for comparable pay onto the employee where length of service is an issue. An employee can claim length of service is irrelevant in their case, but they cannot argue it is irrelevant altogether.

Ms Cadman had claimed that such pay structures unfairly denied women the chance to earn as much as men. She argued that length of service often depended on domestic circumstances such as pregnancy and maternity leave and that employers should have to provide special justification for paying men more than women when they hold the same post and perform the same duties.

The court rejected her claim against the Health and Safety Executive, stating that additional years of service allowed for greater experience which in turn led to improved work performance.

This justified the extra salary paid for length of service even though there was an in-built bias in favour of men who did not take time off to care for children.

Leena Linnainmaa, the president of the European Women Lawyers’ Association, suggested that the situation would only become fairer for women when men took more paternity leave, something most did not do even though they had the right to in most European countries.

“The fact that women take maternity leave is a great burden on their career,” she said.

“We strongly encourage men to take paternity leave and the countries that have no specific legislation on the right to paternity leave to amend their law.”

Emma Hawksworth, a partner in Russell Jones & Walker, which represented Ms Cadman, said: “Additional experience does not lead to better performance indefinitely or in every case. Employers will have to ensure their pay schemes reflect this. Where there is no good reason for using or continuing to use length of service criteria, it will be unlawful to do this.”

Sarah Harman, a leading family solicitor and founder of the pressure group, Families Action for Court Transparency and Openness, said: “To be penalised in terms of salary is a real pity. It seems to go against the run of really encouraging steps in the last 10 to 15 years, which have made women feel that they don’t have to put their children in nurseries and that they can regain the time lost. Length of service and experience are good things, but women who take time off can come back with renewed energy, new ideas, and a different set of skills.”

Bernadette Cadman joined the Civil Service in 1984 and transferred to the HSE as a trainee inspector in 1990. Three years later, she was made an inspector and in November 1996 was promoted to principal inspector.

In 2001, backed by Prospect, she asked to be paid the same as four male colleagues who were in similar jobs.

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