Shares in Sirius Minerals plunged over 20% early on Thursday after the potash development group said its planning application for a project in North Yorkshire remains hung in the balance.The London-listed group has lodged an application to build a materials handling facility and mineral transport system application in the region of the North York Moors National Park.The park's planning committee found there was no ground to guarantee exceptional circumstances to the development project presented by Sirius, adding the economic benefits and compensation do not outweigh the harm caused.However, Sirius said the authorities acknowledged the significant economic and social impact of the project and the strong local support."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," said group chief executive Chris Fraser.Sirius Minerals shares were down 18.41% to 17.54p at 10:07 on Thursday.