About 1,500 care workers are a step closer to sharing up to £50m in equality compensation.
Cumbria County Council appealed against a 2006 ruling that female staff should receive back pay equivalent to bonuses paid to male workers.
But the decision was upheld on Monday by the Employment Appeals Tribunal.
However, the local authority, which has set aside £21m to meet the costs, was given the go-ahead to argue that market rates defined how much staff were paid.
It will make the claim at a further tribunal, likely to be held in Newcastle next year.
Councillor Jack Richardson, cabinet member for equal pay and equal value, said: "The ruling has opened the door for the county council to establish that market rates were instrumental in setting higher pay awards for some employees compared with others.
"This fundamental point of law means that no equal pay claims will be settled until a future tribunal, to which the matter is being referred, examines this issue in depth."
But public services union Unison claimed Monday's decision would pave the way for thousands of cases to be settled.
General secretary Dave Prentis said: "I am delighted that the Employment Appeals Tribunal has ruled in favour of our members. The writing has been on the wall since the employment tribunal decision last year.
"The council should face up to their responsibilities, stop wasting time and more council taxpayers' money on costly legal arguments and give these women the money owing to them."
The exact financial implications of the "enormously complicated legal process" are still unclear, according to the local authority.
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