Shares in Aim-listed Sirius Minerals closed down 13.4 per cent to 18.62 pence per share this afternoon, after a disappointing planning report on its York Potash mine application.
Planning officers at the North York Moors National Park Authority said that the application has "a clear and fundamental conflict with both local and national policies".
Sirius Mineral wants to develop a large polyhalite mine within the park, at a cost of around $3bn (£1.9bn) by the time it completes. Production from the mine would begin around five years after approval is granted.
The company will learn the outcome of its planning application on June 30.
"Responsibility for balancing the various issues related to this application rests with the members of the authority and we believe the case for approval is very strong because the Project represents a once in a generation opportunity," Chris Fraser, managing director and chief exe of Sirius, said.
"As recognised in the officers' report, the York Potash Project can bring significant economic and social benefits to the economies of the local and regional area. The application also includes extensive mitigation, compensation and safeguards."
"We believe our planning case stands up to scrutiny and we remain determined to pursue the positive planning decision that will allow us to deliver this Project and all of its benefits."
Shares in Sirius Minerals were down 16.3 per cent at 18p in mid-afternoon trade.