By Zachary Fagenson

MIAMI (Reuters) - An alliance of shipping interests and a billionaire car dealer launched a newspaper ad on Monday protesting retired English soccer star David Beckham's plans for a soccer arena at the port of Miami, saying it threatens the city's plans to capitalize on the expansion of the Panama Canal.

"We cannot jeopardize well-paying jobs, like crane operators, longshore workers, and mechanics, for low-paying stadium jobs, such as concession sales," the Miami Seaport Alliance said in a full-page ad that ran in the Miami Herald and its sister Spanish-language paper, El Nuevo Herald.

The group, led by John Fox, who has retired as Royal Caribbean Cruise Line's head of governmental affairs, includes two chapters of the International Longshoremen's Association and two stevedore companies, whose workers load and unload ships, along with car dealer Norman Braman, the one-time owner of Philadelphia Eagles.

Beckham last month unveiled detailed plans for a 25,000-seat waterfront stadium on the island port with sweeping views of downtown Miami.

Development of the 36-acre (14.5-hectare) space would cost about $200 million and include shops, hotels and offices connected to the mainland by a pedestrian bridge.

"This is one of four sites under consideration, there's nothing concrete, there's no recommendation pending," Miami-Dade Mayor Carlos Gimenez said on Monday.

The port's master plan calls for the development of more than 7 million square feet (650,000 square meters) of convention, hotel and office space on the same site.

Before Monday's ad, only Royal Caribbean, which is headquartered at the port, has come out publicly against Beckham's plan.

"The plan doesn't interfere with port operations," said Neisen Kasdin, an attorney for Akerman Senterfitt and adviser for the Beckham group. "It will likely generate more revenue for the port in the shorter term than other concepts that have been discussed."

Yet a growing list of opponents say a stadium would jeopardize Miami's aspiration of becoming a more attractive choice for global shippers looking to distribute goods to the U.S. market.

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"There are plenty of other places for the stadium to be," Braman said in a telephone interview.

Miami officials hope the port's short distance from the Panama Canal, as well as $2 billion of planned infrastructure upgrades, including cranes to unload the ships and a $1 billion tunnel connecting the port to major highways, will increase its appeal.

Neither Braman nor Fox would say who paid for the two ads in Monday's paper, worth nearly $25,000.

"I haven't given any dollars yet but I would if asked," Braman added.

Braman spent more than $1 million in 2009 to fight a Miami Marlins' campaign to secure more than $600 million in public funding for a new baseball stadium that opened in 2012.

In 2013, he spoke out against the ultimately failed quest by billionaire real estate mogul and Miami Dolphins owner Stephen Ross to secure $200 million in public funds to help repair the team's arena.

(Editing by Kevin Gray and Mohammad Zargham)