Environmental protesters have spent their first night in freezing conditions in woods near Brecon in the fifth protest against an LNG gas pipe.
Protesters say they will remain in the woodland indefinitely
Despite demos, almost 90% of the first phase of the 190-mile (306 km) pipeline which will run from Milford Haven to Gloucestershire has been completed.
Protesters are occupying trees to stop the second phase beginning.
Despite National Grid's assurances the environment is paramount, the national park has backed the protest.
Campaigners have raised safety concerns and claimed the pipeline, which will carry about a fifth of the UK's gas supplies, will damage the environment.
In November, work on phase one was halted by demonstrators who climbed inside the pipe at Trebanos in the Swansea Valley.
The National Grid is hoping the Department of Trade and Industry (DTI) will give the go ahead for the second phase - to build a pipeline from Velindre to Gloucestershire - by the end of this month.
But around 20 protesters who spent the night in trees and in make-shift shelters on the ground, have said they are prepared to stay indefinitely to stop the work beginning.
"In an ancient woodland where there's habitat that's been there for hundreds of years, they can't replace that habitat unless they've got some sort of time machine," said Nick, one of the protesters.
"You just can't replace an oak tree that's 500 years old."
Brecon Beacons National Park Authority is also concerned about the effect on the environment.
Chris Gledhill said: "We cannot see the justification for the pipeline coming through one of Wales' finest landscapes.
"We are very concerned, and the authority has made it quite clear on many occasions that they do not believe that this is the right route for the pipeline."
But a spokeswoman for National Grid said after months of consultation they had found the "most suitable route".
She said: "We have yet to receive consent for this pipeline which is needed to meet the UK's growing energy need."
She added the land would be reinstated, and that land dug up during phase one was already back in use.