A national park chief has criticised the second phase of a gas pipeline that she claims will damage the area's "special and fragile environment".
Ahead of a six-week public consultation process, Brecon Beacons National Park chair Mary Taylor said she felt an alternative route could be found.
The pipeline will link two liquefied natural gas (LNG) terminals at Milford Haven with the UK network.
National Grid, the firm behind the pipe, said it had consulted the park.
The 115-mile project, which could span the breadth of Wales, will attract new business and industry into Wales, according to National Grid.
On 23 August, the company will publish a list of locations where people can view the plans for the second phase.
Phase one, which runs 120km (75 miles) from Milford Haven to the Neath Valley, is set to be completed in a year.
Phase two - for which planning permission is yet to be granted by the Department of Trade and Industry - would extend further east to Gloucestershire, cutting across a corner of the Brecon Beacons National Park.
Mrs Taylor said: "Our first objective as an authority is to conserve the special and fragile environment of the national park.
"In our view, cutting a swathe the width of a motorway through parts of the national park is simply incompatible with that aim."
She said she understood there were wider energy needs, but stressed that the park "remains to be convinced that the pipeline scheme is unquestionably in the national interest or that there is no alternative to the current proposed route through the national park".
A spokeswoman for National Grid said: "We have been undertaking extensive consultation with the national park, landowners and other interested parties, to come up with the most suitable route for the pipeline.
"We have taken into account various concerns and we believe the route we have chosen provides the best balance of all these aspects."