Developer Sirius Minerals wants to mine near Whitby and build a 23-mile (37km) tunnel to a Teeside processing plant.
Chris Fraser, of Sirius, said its case stood "up to scrutiny" and it was determined to pursue the plan.
The North York Moors National Park Authority is to consider a planning application on 30 June.
The 229-page report by the park authority does not make any recommendation about the proposed mine.
It acknowledged that there were likely to be economic benefits but said the proposal did not represent "exceptional circumstances".
The report said the National Park Authority's statutory responsibility was to "conserve and enhance the North York Moors for the enjoyment of present and future generations".
Sirius said it believed exceptional circumstances had been demonstrated and the mining was in the public interest, with environmental effects "mitigated".
The term "potash" is used to describe a range of minerals containing potassium, and the proposal is to mine polyhalite.
The mine would transform North Yorkshire's economy, Ryedale District Council and Scarborough Borough Council has claimed.
However a report published earlier this month suggested that the five-year construction phase of the mine would cost the area's tourism industry £10m a year.
The Campaign for National Parks said it hoped the authority would reject the planning application.
The decision was an "important test of the protection afforded national parks" and there were strong planning grounds for turning it down, it added.
Plans include a construction of a minehead at Dove's Nest Farm and Haxby Plantation about 1.6 miles (2.5km) from the village of Sneaton involving mining shafts, buildings, roads and a helicopter landing site on a 64-hectare site.

Rafael Pizarro was found in a lock on the Rochdale to Manchester canal in Middleton, Rochdale while playing with friends on 6 June.
He was resuscitated on the towpath before being taken to hospital.
His family said: "Rafael had a big impact on his family and friends and was much loved and we will truly miss him."
Det Insp Kate Atton from Greater Manchester Police said: "Rafael's death is believed to be a tragic accident.
"I would urge people, especially children, to take care around the water and try not to swim anywhere other than supervised swimming pools."
A report into a planned £1.7bn potash mine has said its economic benefits do not outweigh the harm it would cause the North York Moors national park.
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A 13-year-old boy who fell into a Greater Manchester canal in a "tragic accident" has died, police said.