The whole system under which women claim the same wages as men for work of equal value is called into question by a report from the University of Warwick.
Specialists appointed by industrial tribunals to assess the validity of claims take, on average, a year to complete their inquiries instead of the 42 days required by regulations, according to Anne-Marie Plumer, the author of the report.
She says the investigators have 'enormous influence' on cases, but there is 'remarkably little consistency' in the methods they use.
The findings, which form Ms Plumer's MA dissertation, will reinforce criticisms already made by the Equal Opportunities Commission and are likely to influence Gillian Shephard, the Secretary of State for Employment, who is reviewing equality legislation.
There is considerable inconsistency in the choice of factors taken into account when matching jobs, the paper says. In three cases comparing packers and labourers, 'widely divergent' criteria were used by three separate experts. The systems used to award 'scores' to the chosen criteria also varied considerably, as did the weights attributed to each criterion.
Some investigators interpreted 'equal' as 'exact equivalence'; others adopted a more 'broad brush approach'. One woman's job had been rated at 98 per cent of the value of the man's but was deemed 'not equal'. Another was rated at 79 per cent and was found to be 'equal'.
Ms Plumer counsels against 'exact equivalence' because job analysis is not an 'exact science'.
Each of the 24 reports examined - representing 40 per cent of the cases so far - contained a unique method of assessment and thus called into serious question the whole system, the paper says.
Independent experts are appointed from a 12-strong panel serving England, Scotland and Wales, and a 7-member group for Northern Ireland. They are called in when conciliation fails or is rejected.
Ms Plumer says: 'The procedure which produces any one judgement resembles more closely a lottery, dependent on choices which lack rationale, than an objective assessment of worth.'
The equal value legislation was a critical element in the fight for equality because men and women still tend to be segregated into separate jobs. The most recent official figures showed that women's hourly earnings were just 68.2 per cent that of men's.
At the moment comparisons can only be made between people 'in the same employment', but there are proposals that the legislation be extended.
The report concludes that 'until a more consistent method can be produced, industrial tribunals will need to be aware of the influence of choices of methods of assessment can have on the evaluation of jobs and to make appropriate allowances . . .'
Equal Value Judgements: Objective Assessment or Lottery? The Industrial Relations Unit, School of Industrial and Business Studies, University of Warwick, Coventry CV4 7AL.