Sirius Minerals want to build the mine on a site near Whitby.
The firm says the planned York Potash Project will be unobtrusive and lead to the creation of more than 1,000 jobs in the north-east of England.
Business leaders have called it the biggest private investment project in the north of England “by a billion miles”.
But a consortium of 29 campaigning organisations - including the Campaign to Protect Rural England (CPRE) and the National Trust - have joined forces to urge the councillors and other panel members meeting today to reject the proposals, saying the mine was a “huge threat” to the North York Moors and also a “critical test” of the national parks ideal.
Today, about 300 people have gathered for a special planning meeting of the North York Moors National Park Authority at Sneaton Castle, just outside Whitby, where the decision will be made either today or tomorrow.
The scheme has had vocal backing from local MPs Robert Goodwill (Conservative, Scarborough and Whitby), Anna Turley (Labour, Redcar) and Kevin Hollinrake (Conservative, Thirsk and Malton) who wrote an open letter to the committee calling it a “golden opportunity”.
It said: “The York Potash project brings with it the chance to secure a once in a lifetime opportunity to improve the lives of thousands of people in our area.
“A positive decision will bring enormous social and economic benefits to the area by creating jobs, improving training and education opportunities for young people, providing community facilities and by generating more wealth in the economy.”
The MPs said the project “is the single largest private investment in the North for many years and a project of national significance”.
Today, Barry Dodd, the chairman of the the York and North Yorkshire Local Enterprise Partnership said it was the single biggest investment in the north of England “by a billion miles”.
But a range of a groups, led by the Campaign for National Parks have opposed the mine.
In its own letter to the committee, the 29 organisations in the consortium said they were concerned about the implications for all national parks if the mine is allowed.
They also said the project would damage the local tourism economy and have significant transport effects.
The letter asked the committee members whether there is a proven national need for development; whether this development is the only possible site either inside or outside the National Park and whether the harm inflicted on the park by the development can be mitigated?
The letter said: “The Campaign for National Parks believes the answer to all these questions is no.
“We do not consider that York Potash Ltd has provided sufficient evidence to demonstrate the national need. Similarly, there is insufficient evidence that alternative sites outside the National Park have been adequately considered.”
Sirius wants to build on the site, near the village of Sneaton, to access 1.3 billion tonnes of polyhalite discovered below the Yorkshire coastline.
Geologists believe this is the world’s biggest and best quality supply of the valuable mineral.
The proposed site is around 2.5 miles south of Whitby, on the B1416.
The firm says the mine would generate more than 1,000 direct jobs at full production and thousands of indirect jobs in the support and supply industries.
It also said it would create up to 1,800 jobs during construction.
The York Potash Project would see a 1,500-metre mine sunk beneath the park, which would tunnel outwards and pump the polyhalite underground around 30 miles north to Teesside, where it would be processed.
The firm said it believes its project has “massive levels of public support”.