Prince Charles has been urged to block plans for a gas pipeline through part of the Brecon Beacons and three fields on his Duchy of Cornwall estate.
Milford Haven will become one of Europe's biggest LNG terminals
The Countryside Restoration Trust charity (CRT) described the pipeline as a "barmy plan"
National Grid is considering the route to distribute some 20% of Britain's imported liquefied natural gas (LNG) from Milford Haven to Gloucestershire.
The company defended its plans and will announce the route in two weeks' time.
The proposal is part of a venture to turn Milford Haven into one of Europe's biggest LNG terminals and work has already begun in the west Wales port.
Meanwhile, work on the first phase of the pipeline between Milford and Aberdulais, near Swansea, is expected to start next spring.
Robin Page, CRT chairman, said the pipeline's second phase was planned through some of the UK's most attractive-looking countryside, including a 33-mile stretch on the northern fringes of the Brecon Beacons National Park.
It is understood that it could pass through other protected sites along the rivers Tywi, Usk and Wye in Wales.
The CRT is also concerned that the pipeline, to be buried 1.2m underground, will also effect Turnastone Court, its 247-acre farm in Herefordshire's Golden Valley.
"I will also be writing to the Duchy urging the estate to block plans for the pipeline which will go through part of its land," said Mr Page.
"We're concerned with the flora and fauna on our land which is also rich in archaeological relics."
Mr Page said the "beautiful" Brecon Beacons would also be affected.
"The planners have picked one of the most attractive routes in the UK. From every angle it's an absolutely barmy plan," he said.
"I don't know why they don't dock the gas ships in France or Belgium and use existing pipelines."
Caroline Davidson of National Grid said the pipeline was needed because Britain's supplies of natural gas were "depleting".
The pipeline is planned for the northern fringes of the beacons
She added that the LNG gas would be shipped from the Far East to Wales.
"The route on the northern edge of the Brecon Beacons is emerging as the favourable corridor, which will lead on to Herefordshire and Gloucestershire.
"These plans are yet to be finalised".
She said National Grid would restore the countryside after the pipeline had been laid.
A spokeswoman for the Prince of Wales said the proposal could affect three fields belonging to the Duchy of Cornwall estate.