The Corrections Association has been denied leave to appeal against an Employment Court judgment in favour of extending the use of double-bunking in jail cells.
Four prisons are being double-bunked to cope with an increase in the prison population.
The union earlier argued that increasing the use of such cells breached its collective agreement with the Corrections Department.
In December, the union lost a legal fight about the issue in the Employment Court.
Now the Court of Appeal has also dismissed the case.
A 12-page judgment says the proposed appeal does appear to raise genuinely arguable questions of law.
But it says those are matters of interpretation of the collective agreement between the union and department, which the court cannot rule on.