The GMB general union sacked 41-year-old Frances Kidd from her job as a book-keeper after she co-operated with a police investigation.
Mrs Kidd - who received death-threat phone calls and suffered a barrage of abuse from Militant shop stewards - wants her old job back.
Her ordeal began in 1991 when she voted in favour of redundancies among council workers, members of the union.
Her angry bosses refused her leave to perform council duties - then sacked her for 'gross industrial misconduct' after she took a detective to the union office in Liverpool to check evidence in a court case.
But an industrial tribunal issued its written decision yesterday ruling that she had been the victim of unfair dismissal by the union.
Its panel said: 'We are satisfied that the decision to dismiss was one that fell outside the range of reasonable response open to any reasonable employer.
'We found that, in the circumstances, that having regard to equity and the substantial merits of the case and the size of the respondant's undertaking and administrative resources they had acted unreasonably.'
Rex Makin, Mrs Kidd's solicitor, said that the union should now set an example of conciliation to other employers by re-instating her.
If Mrs Kidd and the union do not reach a settlement by 12 October the tribunal will reconvene to award compensation.Reuse content