The Brecon Beacons National Park Authority is to oppose plans by the National Grid to lay a major gas pipeline under the park.
The planned route passes through the Brecon Beacons
The planned pipeline would bring gas from new liquefied natural gas (LNG) terminals at Milford Haven, west Wales, into the existing gas network.
Some of the pipeline's 115-mile route would pass through the Brecon Beacons.
However the park authority said at a meeting on Tuesday it would challenge any application for consent.
The National Grid Transco utility company had made a presentation to the authority at an emergency meeting on Tuesday, detailing where the pipeline would go and what the environmental impact would be.
Following the meeting, the park authority said it would oppose any application by the National Grid to the Department of Trade and Industry to lay the pipeline through the park.
In a statement, the authority said the pipeline would run directly into a newly-designated section called the Geopark, containing "some of the most spectacular geology in the world".
Authority chair Margaret Morris said: "We are concerned over the impact this will have on the many communities and businesses along its entire length, not just within the park itself.
The Beacons contains some of Wales' most beautiful countryside
"We know from the approaches already made that people are very concerned."
Chris Gledhill, the park's chief executive, added: "We have no objection to the original plans to build a pipeline between Milford Haven and Aberdulais - indeed this is what National Grid explained to us was the original concept.
"It is the route of this pipeline and the very need for the pipeline beyond Aberdulais that we are objecting to, and we do not believe that Transco has demonstrated an unequivocal case for either."
The new pipeline network is intended to transport natural gas from two LNG terminals currently under construction at Milford Haven.
Liquefied gas will come into the port in tankers and will be turned back into gas before entering the pipeline network.
The first pipeline will run from Milford Haven to the Swansea area.
Work is under way on LNG terminals at Milford Haven
A second pipeline, beginning at Felindre, near Swansea, would pass through 16 miles of the Brecon Beacons national park before heading towards Hay-on-Wye and ending at Tirley, Gloucestershire, where it joins the existing gas network.
According to the National Grid, which provides Britain's gas infrastructure, the pipeline, which is buried 1.2 metres underground, will transport around 20% of the gas needed to meet UK consumption.
There have been some concerns raised about the pipeline's impact on some farming land and on archaeological sites.
The Brecon Beacons National Park Authority, which administers the national park, has previously said it would attempt to minimise the pipeline's impact.
A statement said: "We cannot prevent the pipeline passing through the national park [but] we will do all we can to minimise how much of it comes into the park and work hard to ensure no lasting damage to the beauty, wildlife, cultural heritage and farming practices of this national treasure."
The National Grid has said it plans to use engineering techniques to minimise the effect on the environment.
It is expected that more than 1,000 people would work on the project at the peak of construction.