The Brecon Beacons National Park has lost a High Court challenge over a planning appeal which gave the go-ahead for a green energy plant.
A Welsh Assembly Government planning inspector said in January that an anaerobic digester could be built in Talgarth, near Brecon.
But the park went to the High Court in Cardiff seeking clarification as to how the inspector applied planning policy.
The park authority said it was disappointed by the judgement.
The digester was the idea of farming brothers Paul and Gary Jones, and it is supposed to turn waste from a local abattoir into gas. The waste rots down and is then spread on fields as a fertiliser.
The Jones family submitted a planning application for its diversification project to the national park in September 2007.
It was awarded permission in 2008, but that was withdrawn last year.
The family appealed against the decision and a public inquiry was held. The assembly government's planning inspector then said the project was free to proceed.
The park authority lodged a challenge in the High Court in the weeks after the inspector's decision, saying it "did not explain the way in which the inspector applied planning policy".
After the court hearing, national park solicitor Rhiannon Edwards said: "Brecon Beacons National Park Authority is disappointed with today's judgement, but will give it careful consideration when dealing with future applications.
"This challenge was not undertaken lightly. We believe that this is the first anaerobic digester of this scale to be approved in any UK national park under farm diversification policies.
"Given the uncertainty surrounding this scheme the national park authority had little option but to seek clarification from the court.
"Today's judgement has provided further guidance that will be helpful in considering similar applications in the future."
Farmer Paul Jones said: "I'm really pleased with the way the case went, but I feel it's another year wasted and a waste of taxpayers' money.
"We're planning to start work on the digester as soon as possible.
"I'll be seeking legal advice over the handling of the planning application by the national park, but I feel it is time to move on."