The planned route of a 115-mile gas pipeline running across some of Wales' protected countryside has been revealed by utility company the National Grid.
The underground pipeline will transport gas from new liquefied natural gas (LNG) terminals at Milford Haven, west Wales, into the existing gas network.
It will run from Felindre, near Swansea, to Tirley in Gloucestershire, via the Brecon Beacons and Wye Valley.
Countryside bodies have called for areas on the route to be protected.
The pipeline is the second phase in the construction of a network 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 new pipeline network.
The first pipeline will run from Milford Haven to the Swansea area.
Work is already underway on LNG terminals in Milford Haven
The second pipeline, buried 1.2m underground, will end at Tirley, Gloucestershire, where it joins the existing gas network.
According to the National Grid, which provides Britain's gas infrastructure, the pipeline will transport around 20% of the gas needed to meet UK consumption.
National Grid Project Manager David Mercer said North Sea gas reserves were running down and told the BBC the pipeline was "key" to maintaining a secure gas supply for the UK.
He said identifying a route had been an "extremely lengthy and complex" process.
Mr Mercer added: "The preferred corridor provides the best balance between environmental impact, land use and construction requirements.
"As far as possible we avoid areas of population, taking into account environmental, geological, and archaeological features and construction, health and safety considerations."
Before finalising the route, the National Grid said it would work closely with landowners to agree the best route and would use engineering techniques to minimise the effect on the environment.
It is expected that more than 1,000 people will work on the project at the peak of construction.
The Brecon Beacons National Park Authority, which administers the national park, said it would attempt to minimise the pipeline's impact.
A statement said: "It almost goes without saying that we think pipelines like this should not go through national parks.
The pipeline will run through part of the Brecon Beacons
"Unfortunately the legislation that protects national parks...does not preclude some development of overriding national interest.
"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 that no lasting damage to the beauty, wildlife, cultural heritage and farming practices of this national treasure."
Sioned Humphries, from the Campaign for the Protection of Rural Wales said parks should have "special privileges" as "extremely important landscapes".
She told the BBC: "We're worried about the fact it still runs through the national park and also through three rivers of European importance - and a number of sites of scientific interest."
The Country Landowners' Association said more than 500 farmers and landowners would be affected and it had "enormous concerns" about the disruption that would be caused.
Peter Davies, from the Farmers' Union of Wales, also acknowledged there would be a "huge impact" on the farming community.
But he added: "Provided they work with the owners of the land, provided the compensation is fair then the good will is there."
Map of pipeline's route