Only last week Carnival became the second major cruise ship operater to admit it had polluted the oceans, agreeing to pay a whopping $18 million fine in a Miami court.

A spokeswoman for Royal Caribbean confirmed a writ had been issued on Wednesday but said the company could not comment until it had seen the law suit.

"We have not seen the law suit yet, but I am certainly aware of it," she said speaking from Miami.

Carnival Corp said it was also aware of the law suit but could not as yet confirm that the writ had been filed.

"We have a copy of the complaint from a third party but our California attorneys have not been able to confrim that it was filed," vice-president of public relations Tim Gallagher told Reuters from Miami.

A spokeswoman for P&O Princess said it was checking.

"Cruise Lines must stop fouling California's waters in the name of good times and big profits," said Bluewater Network's campaign director Teri Shore.

The four plaintiffs, that include the Environmental Law Foundation, the Sufrider Foundation and San Diego BayKeeper, allege the discharge of ballast water violates a Californian law passed in 2000.

Ballast water, used to maintain ship stability, sometimes contains exotic organisms that pose a threat to native flora and fuana.

The environmental groups say that water is the single largest source of invasive species found in California ports. Ships usually take on ballast in foreign ports only to dump it elsewhere.

The lawsuit claims that in two out of three visits, ships owned by Carnival, Holland America, Princess, and Royal Caribbean violate a ballast water law passed in 2000.