A proposal to build a 25,000-seat soccer stadium on a neglected corner of PortMiami is drawing opposition from a group of port users, including Royal Caribbean Cruises Ltd.

The planned stadium would be adjacent to RCCL’s headquarters campus at the port.

A coalition called the Miami Seaport Alliance was recently formed to oppose the stadium, which would house a new professional franchise partly owned by soccer superstar David Beckham.

The Miami-Dade County Commission, which governs the world’s busiest cruise port, voted in December to have the county manager begin negotiations with Beckham’s group.

There are several other non-port sites under consideration, but Beckham prefers the port site for its views of the downtown skyline.

Renderings of the proposed stadium have been created by Arquitectonica, the Miami firm that designed the nearby American Airlines Arena, home to the Miami Heat professional basketball team.

According to John Fox, president of the Miami Seaport Alliance, the area around the port entrance is already congested.

“When there’s a Heat game, or things going on at the Arsht Performing Arts Center, we now have an art museum that’s fantastic, there’s a children’s museum there, there’s a science museum coming,” Fox said. “And so, even forgetting the cruise and cargo interests, there’s horrendous, horrendous traffic problems.”

Currently, the only way to access Dodge Island where the port sits is over a bridge from downtown Miami, though a $1 billion tunnel connecting the port to a freeway is nearing completion.

Fox, who is the former vice president of government relations at RCCL, said an open-air stadium is simply not an appropriate use of the port.

“Is there another seaport in the world that would even consider putting a sports arena with 30,000 fans right in the middle of its public property?” he wondered.

Beckham’s real estate adviser has said that games would occur at night, after cruise ships have departed. But Fox said there was no way a single team could guarantee the league’s schedule.

The stadium would take about 12 acres on the port’s southwest corner. There is currently a warehouse on the parcel used by the Marine Spill Response Corp. The port’s master plan calls for development of office towers, hotels and other such uses there.

Wary of competition, Miami’s Downtown Development Authority is supporting the Beckham stadium, which might also include ancillary development but be far less intensive.

Fox said his group includes seven or eight members, including another cruise line besides Royal Caribbean, labor unions, stevedore companies and freight forwarders. Two other cruise lines, Carnival Cruise Lines and Norwegian Cruise Line, have said they’re not involved.

The closest cruise terminal to the proposed stadium is Terminal J, which is used by Oceania, Regent Seven Seas, Crystal and Azamara.

Fox said that if the stadium were to be built and make access to PortMiami more difficult, it could mean an exodus of cruise ships and passengers to other East Coast ports, particularly Port Everglades 30 miles to the north.

“The [port] users are concerned that this will either choke off the economic engine or bring it to a halt,” Fox said, though he added that the decline would be gradual.

“This isn’t going to happen overnight,” he said. “But as businesspeople … make decisions about where to put their freight or their cruise lines, certainly this is one of the things that needs to be considered.”