Streetlights placed at a dark sky reserve where people travel to view the Milky Way and major constellations have left a campaign group concerned.
The Brecon Beacons National Park had the night sky above it protected to prevent light pollution in 2013.
LED Street lighting was installed in Cwmdu, Powys, by the Welsh Government, for "driver and community safety".
International Dark Sky Association (IDA) said this was not consistent with the ideals of dark sky preservation.
Older street lights have been replaced by LED units on the A479, which as a trunk road means the Welsh Government does not need permission from the national park authority.
John Barentine, a spokesman for the non-profit IDA, said he was worried about the implications of the development.
While there are no current plans to take away the status, he said the situation would be monitored closely.
"The information I have available to me suggests that the type of roadway lights installed and operated in Cwmdu are not consistent with the notion of Brecon Beacons as having an 'exceptional commitment to, and success in implementing, the ideals of dark sky preservation'," Mr Barentine said.
He said park staff were surprised when the Welsh Government installed the lights.
While admitting it had done nothing wrong, he said the move could lead to more lights being installed in the Brecon Beacons and other national parks in the UK.
Mr Barentine said the design and effect of the lights were "inconsistent with both these goals and the protected status" of an international dark sky reserve.
He added: "The government has been generally quite supportive of dark skies efforts across Wales, which is why the new installation of lights in Cwmdu is perplexing.
"We would like to work with the government to find a way to mitigate any adverse impacts of these lights."
A Welsh Government spokesman said: "As part of its commitment to reduce its energy consumption and carbon footprint the Welsh Government, during the course of maintenance, recently replaced old lanterns at this location with modern LED units.
"The intention was not to have any consequential adverse effect on the environment and options such as dimming in the vicinity of Cwmdu campsite will be considered provided there are no safety implications."