Trainer Mick Ruis is suing the California Horse Racing Board alleging it intentionally failed to discharge its public duty when it hid and subsequently dismissed Justify's positive drug test in the 2018 Santa Anita Derby.

Ruis trained and owned Bolt d'Oro who finished second in the race, the West Coast's major lead-up to the Kentucky Derby.

Santa Anita Derby winner Justify, trained by Bob Baffert, went on to sweep the Kentucky Derby, Preakness and Belmont Stakes and become horse racing's 13th Triple Crown winner that year.

The petition filed in Los Angeles County Superior Court on Monday asks for a writ ordering the CHRB to set aside its decision to dismiss Justify's positive test in the Santa Anita Derby and to order the horse's disqualification with a redistribution of the purse in the $1 million race.

Justify earned $US600,000 for the victory while Bolt d'Oro received $US200,000 for second.

Justify tested positive for scopolamine, a prohibited substance detected in his post-race urine sample. The substance is used to treat mild colic and spasms but also can result from eating jimsonweed.

Under California rules at the time, a failed test for the substance would have required disqualification and the redistribution of prize money. That would have resulted in Bolt d'Oro being declared the winner.

The CHRB has since lowered the penalty for a scopolamine positive from the tougher 3b classification at the time of Justify's positive, which required a fine and disqualification, to a 4c that includes a minimum fine of $US1000 and no disqualification.

Lawyers for Ruis contend that rule change did not officially occur until January 1, 2019, months after the test was ruled positive.

The petition alleges that the CHRB decided in a closed-door session of its board to dismiss the matter against the horse and Baffert.

The positive result did not become public until last September when it was revealed in a New York Times story.

CHRB equine medical director Dr Rick Arthur has said evidence suggested contamination rather than an effort to skirt the rules.

He said that because California has had issues with jimsonweed contamination, it had a higher threshold than the international limit for scopolamine.

Justify is about to enter his second season at Coolmore's Ashford Stud in Kentucky after standing at Coomore Australia for the southern hemisphere season.