The airfield was built in 1938 and used during World War II
The three national park societies of Wales are teaming up to try to stop an airport opening in Snowdonia.
The former military airfield at Llanbedr near Harlech, Gwynedd - in the heart of the national park - is set to be taken over by a private airport.
But the Snowdonia Society, Brecon Beacons Park Society and Friends of Pembrokeshire National Park say commercial planes will ruin the area.
The Welsh Assembly Government said the scheme was "in sympathy" with the area.
The RAF military airfield was built in 1938 and used during World War II.
It was decommissioned by the Ministry of Defence in 2004 and acquired by the assembly government in 2006.
It has been taken over by Kemble, which already operates a private airport in the Cotswolds.
The new operator is planning an open day and mini-air show on 20 August to launch the new venture.
But the three park societies are pressing for the airport project to be put on hold until there has been a full and public examination of the proposal - something they claim has not happened.
They claim that complex planning rules mean the airport is able to re-open without being scrutinised.
"The airport would pose a threat to the environment and tourism, which is the number one employer in the area and we're very concerned about it," said Alun Pugh - a former culture minister - who is the Snowdonia Society's director.
"Every year, thousands and thousands of people come to Snowdonia for peace and quiet - they don't want to hear aeroplanes buzzing overhead. They could go to Heathrow Terminal 5 for that.
"On top of that, ministers have a statutory duty [under the Environment Act] of 'conserving and enhancing the natural beauty, wildlife and cultural heritage'."
Nigel Phillips, chair of the Brecon Beacons Park Society and Brian Powdrill, chair of Friends of Pembrokeshire National Park, joined Mr Pugh to write a letter to The Guardian newspaper to voice their concern.
In it they claim assembly government ministers have failed to take the special status of national parks into account before making any decisions on land use.
Mr Phillips said: "We wanted to show our support for Snowdonia over this.
"It seems as if the airport is trying to be pushed through. It's complicated in terms of planning regulations because there's the issue of whether a commercial airport would constitute a change of use from a military airport.
"The airfield was built during World War II and was given crown immunity because it was in the public interest.
"That crown immunity seems to be being passed down and the issue has not been scrutinised.
"What we are fighting for now is peace and tranquillity in our national parks."
An assembly government spokesman said the scheme would have "minimal environmental impact" with the emphasis on its "sustainable development in sympathy with the local area and community".
"The development is essentially a continuation - albeit at a significantly reduced level - of the activities previously carried out at the aerodrome," he said.
"As a military airfield, the base had seen a relatively high level of flying activity, with around 6,500 air movements recorded in its last year of operation alone."
He said all relevant authorities and bodies had been kept aware of the development.