The proposed site for Inter Miami's stadium has been found to have dangerous levels of toxicity.
David Beckham's side will join MLS in 2020 with its new stadium opening one or two years later.
Melreese Country Club golf course has been proposed as the site for the stadium but an environmental study has highlighted serious problems for Beckham and the club.
Miami Herald reports the analysis has shown levels of arsenic contamination have reached more than twice the legal limit.
It is also claimed surface-level soil samples contain 'physically hazardous debris among other contaminants.
Beckham's Miami Freedom Park project, that will also see a commercial complex on the 131-acre site, has already reached a cost of $1bn (£821.7m).
And it is now feared the cleanup could cost a further £41m ($50m) to the former England captain.
Miami Mayor Francis Suarez said: "This obviously causes great concerns. We’re glad this event has caused us to test the site.”
“Basically, the site has significantly more contamination than what is commercially reasonable."
Beckham hopes to build a 25,000-capacity stadium on the site for the MLS side.
A hotel, office park, shopping centre and rooftop football pitches are all proposals for the site.
Inter Miami so far have three registered players with Beckham making a move to bring in some big names ahead of next season.
Cristiano Ronaldo , Edinson Cavani and Gerard Pique have all reportedly been identified as potential signings.
Mirror Sport revealed earlier this month that former Manchester United boss David Moyes was in contention to manage the team, as are Carlo Ancelotti and Thierry Henry.
