Bristol City FC's plans for a new £92m stadium have received a blow after a planning inspector supported a move to declare the site a "town green".
About 200 people backed the legal bid to prevent new development on the 42-acre site in Ashton Vale, close to the the club's current Ashton Gate ground.
An independent inspector concluded the whole site met the legal requirements and said status should be granted.
The final decision now rests with Bristol City Council.
The authority has already approved the planning application for the stadium.
Land can be registered as a town or village green under the Inclosure Act 1857 and Commons Act 1876.
Applicants have to prove the site has been freely used for recreational purposes for 20 years or more.
An inquiry into the application opened in May at the council house. Town green status would also put an end to the possibility of Bristol hosting World Cup games in 2018.
Bristol City FC's chief executive Colin Sexstone said they were "naturally disappointed" by the report.
"We always knew that prizes on the scale of a new regional stadium and a World Cup were never going to fall into our lap - the most ambitious and visionary projects never do - but we remain utterly determined to get there," he said.
A spokesman for Ashton Vale Heritage Group said town green status would protect the land for future generations.
"Local residents have been protecting these fields and fighting development of this site for more than 40 years," he said.
"If we have achieved town green status, then we hope once and for all we can be free of the anxiety and stress we have endured for the last two years."
Councillor Simon Cook, deputy leader of the city council, said he was "shocked and disappointed" by the inspector's decision.
He said: "The new stadium, a possible arena, redevelopment of Ashton Gate and Bristol's World Cup Host City status are at serious risk.
"This amounts to around £150m worth of investment, promising at least 1,000 extra full and part-time jobs and thousands of construction jobs.
"It also means we risk losing the economic impact of hosting the World Cup amounting to possibly £150m.
"It would be devastating to the regeneration of south Bristol and highly damaging to the image of the city."
He said the authority was "determined to chart a positive way forward", so the interests of people having an open space to walk their dogs were balanced with "the wider economic benefits".
The inspector's conclusion will be considered by the council's public rights of way committee in due course.