A Seattle port workers union has appealed a court ruling that a $490 million sports arena planned to house the city's sorely missed National Basketball Association franchise does not violate Washington state environmental laws, the union's lawyer said.

The proposed stadium is key to a plan by a prominent Seattle investor to buy the Sacramento Kings and move them to Seattle renamed the SuperSonics to replace a team of that name which the city lost to Oklahoma in 2008.

A Seattle judge dismissed a lawsuit last month that had sought to block plans to build the arena south of downtown, in a ruling that cleared a major hurdle for the investor group aiming to buy the Sacramento Kings.

The original lawsuit, brought by a local longshoreman's union, argued that the backers of the proposed stadium had violated state environmental laws because they did not complete an environmental impact statement before striking a deal last year to build a multi-use arena adjacent to the city's port.

Attorneys for the union argued that a traffic influx from the proposed arena would stifle transportation corridors leading in and out of the port, one of the busiest in the United States.

"The decision makers must have the environmental information in front of them before they start moving down the path," David Mann, a lawyer for the local branch of the International Longshore and Warehouse Union, told a news conference.

Attorneys for the city of Seattle, King County, and lead investor Chris Hansen's development company, WSA Properties III, had successfully argued that their "memorandum of understanding" to build an arena did not represent an action that required an environmental impact statement, which is being completed anyway.

King County Superior Court judge Douglass A. North sided with the stadium backers and rejected the union's lawsuit, saying there had not been a legally binding action by the arena's backers that would require an environmental impact statement.

Assessments of both the environmental and economic impact of the proposed arena are under way, though port workers worry they are too limited in scope and that the specific plan to build the arena on land Hanson purchased is a foregone conclusion.

"Our livelihoods, our jobs, and the future of maritime jobs in Seattle are in jeopardy with the ... arena," said Cameron Williams, the president of the local union.

The proposed stadium deal, which has local support but still needs final approvals, calls for $200 million in public funding to be paid back by arena-related revenues.

Hansen and his co-investors - including Microsoft Chief Executive Officer Steve Ballmer - have struck a deal to buy the Sacramento Kings. Several reports have put the value of the deal at about $340 million, but the sides have not commented publicly on the size of the agreement.

The deal also requires approval by the NBA and was met with a challenge last week from investors recruited by Sacramento Mayor Kevin Johnson.

Johnson said on Thursday that 24 Hour Fitness founder Mark Mastrov would bid to buy the team from its owners, the Maloof family, and that billionaire supermarket mogul Ron Burkle would lead an investor group to build a new downtown arena.

Attorneys for the union said they filed the appeal on Monday with King County Superior Court, which then transfers it to the court of appeals in Seattle. (Reuters)