The Department of Trade and Industry (DTI) has given the go ahead for the second phase of a project to build a natural gas pipeline through Wales.
Anti-pipeline protesters in a tree in January
The 122 mile (196km) section will pass through part of the Brecon Beacons.
The national park authority called it a "huge blow" but the DTI said "stringent conditions" would minimise impact on the environment.
Work will start between Felindre, near Swansea, and Tirley in Gloucestershire at the end of the month.
The National Grid said the route was chosen carefully after consultation.
It passes close to Ammanford, Llandeilo, Brecon and Hay-on-Wye, before heading out towards Ross-on-Wye.
The pipeline is expected to start operating in October.
But the project has already attracted opposition.
In Trebanos in the Swansea Valley there have been several protests, and recently five people were charged with aggravated trespass.
Last month, environmental protesters braved freezing conditions to campaign in woods near Brecon.
But despite the demonstration almost 90% of the first phase of the 190-mile (306 km) pipeline has been completed.
National Grid, the firm behind the project, said the pipeline would provide more than 20% of UK gas requirements, helping to meet the country's growing demands and address declining North Sea reserves.
The pipeline will run from Pembrokeshire to Gloucestershire
"We are pleased that the Department of Trade and Industry has acknowledged the vital importance of this pipeline in maintaining gas supplies to the UK," said National Grid's senior project manager David Mercer.
He said the route was chosen carefully after a discussions with landowners, the national park and assembly government agencies such as the Countryside Council for Wales.
Mary Taylor, chair for Brecon Beacons National Park Authority, said: "Naturally, we are extremely disappointed at today's decision.
"Throughout the entire pipeline process, we have maintained a clear and consistent position that we do not believe it is appropriate to route a gas pipeline through the national park."
Chris Gledhill, the park authority's chief executive, said the decision was a "huge blow".
Announcing consent for the project, energy minister Lord Truscott said the potential impact of the pipeline on the beacons had been "uppermost" in his mind.
"My officials have applied rigorous assessment criteria and this process has been instrumental in enabling the company find a route which avoids the Blaenavon world heritage site and halves the length of pipeline going through the national park."
He said "stringent conditions" on National Grid would enable them to deliver the project "on time but with the least possible environmental impact".