In a row that is dividing environmentalists, Dr Adam Watson, a conservationist and scientist, has called for a public inquiry into the project and the removal of any trees that have already been planted.
Opinion is divided over RSPB Scotland's plans to almost double the total size of the woodland at its Abernethy Forest, near Grantown-on-Spey in the heart of the national park.
The bird charity wants to join it up with the fragmented surrounding remnants of ancient Caledonian pine forest.
Although the main component of Caledonian pine forest is the native Scots pine, a critical element of ancient pine forests include a broader range of native shrub and broadleaved tree species - such as juniper, birch, rowan, alder and willows.
Whilst recovery of the pine element at Abernethy has been successful, some of these other species remain extremely scarce so they will be part of the project.
But Dr Watson, who served on the Cairngorm Partnership which laid the groundwork for the Cairngorms National Park, has attacked the plan, Scottish Natural Heritage (SNH) and Forestry Commission Scotland (FCS).
He said: "SNH and FCS are statutory guardians of Caledonian woods. Regulations stipulate natural regeneration, which has continued for millennia, free. Expensive human interference by collecting and growing seeds involves unnatural selection, so that planted trees differ from original natural populations."
Dr Watson, who lives on Deeside, continued: "The RSPB assert that planting is needed because broadleaved trees are scarce. This belies profound ignorance of Caledonian woods and their north-European counterparts. These species are scarce because the less acidic soils that favour them are scarce. We should respect nature, not think arrogantly we know better."
He said SNH's consent to RSPB planting involved local staff who lacked recognised pinewood expertise and did not consult SNH's senior scientists.
He claimed that by consenting, FCS also broke the regulations.
"SNH and FCS knew this last autumn but divulged nothing publicly. SNH, FCS and the Cairngorm National Park Authority neglected their duty. This requires public inquiry; withdrawal of consent to plant; and removal of trees planted so far."
But the Forestry Commission said the proposed planting was the subject of a detailed planning and consultation process and is mainly comprised of poorly-represented broadleaf trees found locally.
But SNH said: "There is no regulation stipulating natural regeneration in old Caledonian pinewoods. Planting is in fact an important tool in woodland expansion. These RSPB proposals, which we fully support, have been through very thorough assessment.
"This is a valuable long-term plan which will greatly enhance native species and local biodiversity."
Nobody was available at the park authority, while RSPB Scotland declined to comment.
But Dr Watson has support from other critics. Basil Dunlop, a former chief forester and author of The Regeneration of our Native Pinewoods, who sat on committee or boards with SNH, FCS and the Cairngorms National Park Authority.
He wrote to The Herald last week calling for respect for the ancient Caledonian forests ".. to accept that this woodland should be replenished by natural regeneration as it has been since the last Ice Age.
"Planting trees in this location is landscape gardening on a massive scale, not conserving our natural heritage."
However, directors of Woodland Trust Scotland and the Trees for Life charity have backed the RSPB's plans.