Litigation over the Golden State Warriors’ planned Mission Bay arena will delay the opening of the facility for a year, the team said Friday.

The Warriors now expect to spend much of this year fighting lawsuits brought by the Mission Bay Alliance, a well-funded group of UCSF donors who say the arena will have a negative impact on the UCSF Medical Center at Mission Bay, which opened in November 2014.

The legal fights mean the Warriors will likely open the 2019 season in Mission Bay, not the 2018 season as planned.

“The only thing this lawsuit will accomplish will be to waste everyone’s time, delay all the jobs and economic activity the arena will bring and line the pockets of a bunch of lawyers,” team spokesman P.J. Johnston said. “Since we’re a basketball team, we have to think in terms of seasons. And the fact is, if we have to fight this thing in court over the course of 2016, we likely will be delayed by a season.”

Late last year, the San Francisco Planning Commission and Board of Supervisors unanimously approved the $1 billion arena. It has won the endorsement of UCSF administration as well as biotech industry executives in the neighborhood.

So far the Mission Bay Alliance has filed two lawsuits against the project. One charges that UCSF Chancellor Sam Hawgood overstepped his authority by signing a memorandum in support of the project in exchange for transportation improvements pledged by the city of San Francisco. In doing so, the complaint filed in Alameda County Superior Court says, Hawgood tied the hands of the University of California without the approval of the system’s Board of Regents.

The other lawsuit, filed in Sacramento County Superior Court, argues that the city violated state environmental laws by not properly considering alternate locations for the arena and by failing to adequately address the project's environmental impacts, such as traffic, air quality and noise.

“This is a real victory for the opponents of the Warrior’s ill-conceived Mission Bay arena,” said Sam Singer, a spokesman for the Mission Bay Alliance. “Mission Bay Alliance’s two lawsuits, along with growing public opposition, is going to lead the Warriors to more delays, and we believe that it will ultimately cause the team to depart the Mission Bay site.”

This marks the second time the Warriors have pushed back the opening of a new San Francisco arena. Originally the team hoped to build a waterfront arena on Piers 30-32 and were targeting a 2017 opening. That date slipped a year when the team, facing neighborhood opposition, switched its focus to Mission Bay.

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Christine Falvey, spokeswoman for Mayor Ed Lee, said the news of the delay was “disappointing, though not unexpected.”

“Remember, this is the same anonymous group that said they will litigate until the cows come home,” she said of the alliance. “While there is a season’s delay for our residents and the many workers who expect to work on and in the arena, at the end of the day, we will have a state-of-the-art, privately financed arena for generations to come.”

Vic Tafur and J.K. Dineen are San Francisco Chronicle staff writers. E-mail: jdineen@sfchronicle.com, vtafur@sfchronicle.com Twitter: @VicTafur, @SFjkdineen