Campaigners have described a scheme to build a marina and 200 parking spaces on an island in Poole Harbour as a "disaster".
The Crown Estate, which owns the sea bed around the harbour, favours a plan for a 600-berth marina and 200 car parking spaces by marina group MDL.
Campaigners say the environmental impact of the scheme has been ignored.
The Crown Estate says consultation and environmental impact assessments will be made in due course.
If the development goes ahead campaigners say the car park would be visible from the shore and National Trust-owned Brownsea Island.
The new marina would also be situated close to protected wetland and sites of special scientific interest.
Mark Howell of the Poole People Party said the development would be a "disaster" for the harbour.
In a statement, the Campaign to Protect Rural England (CPRE) said Poole councillors learned of MDL's plans on 12 December. It also said Poole fishermen had not been consulted.
Terry Stewart of CPRE said: "We're staggered at the degree of urgency on all of this by the Crown Estate, and they want to make a decision, irrevocable, by 14 January.
"We haven't seen any plans, maps or technical details."
By contrast, the rival bid by Poole Harbour Commissioners to build a 900 berth marina and public walkway, a viewing platform and visitor centre has been under consultation for two years, with more than 65 public presentations and consultations.
In a statement, a spokesman for the Crown Estate said although MDL was currently the preferred bidder, the marina group was currently working to address a "number of issues" relating to its proposal, and the situation would be reviewed in the new year.
He said any future leasing agreement for use of the seabed would be subject to consent from Poole council, Poole Harbour Commissioners and the Marine Management Organisation, and added "appropriate public consultation will form part of the consenting process".