The Cromarty Firth Port Authority (CFPA) has criticised Global Energy Group's proposal to create its own port on the shores of the firth.
CFPA has concerns that the new harbour would control a large section of the main access route in and out of the Cromarty Firth.
Global Energy Group has not commented on the criticism.
The energy industry service firm owns the Nigg fabrication yard on the shores of the firth.
CFPA operates the firth's long established port at Invergordon.
Invergordon and the Cromarty Firth serve cruise liners and the North Sea oil and gas industry.
CFPA chief executive Bob Buskie said his 40 year old organisation was a trust set up by an act of parliament and acted as custodians for the firth.
He said a director at Global Energy had confirmed the company's plans in a letter.
Mr Buskie said: "As the custodians of the Cromarty Firth for the public, we have huge concerns what these plans would mean for the cost of access to the firth, for navigation and for the health and safety and natural environment in these waters."
Plans for a private harbour on a Highland firth threaten access to the area, a port authority has said.