An open survey of farmers, residents and businesses affected by the Galloway National Park plan found three-quarters of all respondents were opposed to it.
Nearly 2,000 responded to the NFU Scotland poll, showing that a clear majority (73%) were against the proposed national park bid.
The results, from the most extensive survey in the proposed park area to date, showed that just 16% were for the proposal and 11% were undecided.
Only 28% of 2,000 respondents were union members and, in total, 33% of people identified themselves as farmers.
If given the official designation, Galloway will be the first national park in the UK in about 15 years.
But NFU Scotland said the survey results gave it a clear mandate to continue opposing the proposal, which it said had the potential to impact on farmers and the wider rural economy.
NatureScot has been charged with helping the Scottish government decide if Galloway should be given national park status and what geographical area it should include.
But NFU Scotland’s chair of Dumfries and Galloway region, Stewart Wyllie, said that with almost 2,000 responses, the majority were 'clearly against' the plan.
“NFU Scotland continues to meet with NatureScot to establish how it will fulfil its remit as reporter and ensure the views of all are considered," he explained.
“[We] will be using the consultation process to underline our continued opposition and work in expressing these views to NatureScot, Scottish government and wider stakeholders.”
Proposals for the new national park were recently criticised as being 'unsound' by rural business organisation Scottish Land & Estates (SLE).
Its own survey, published last week, found that 91% of rural businesses, estates and farmers located within the proposed park boundaries opposed to the plan.
Stephen Young, director of policy at SLE, said: “What has become clear is that there is very little support from land-based businesses in Galloway and they are the ones who will be most affected by a new national park being formed.
“In our view, the unsound proposals fail to demonstrate how the creation of this national park will deliver positive outcomes for people, jobs and nature."
In November, there will be a formal 12-week public consultation on the proposal.
It will look at the detailed arrangements for a national park, including its area, powers and governance.
