By Nick Brown 

Jan 22 (Reuters) - The U.S. government has joined a lawsuit from college and professional sports leagues seeking to stop New Jersey from implementing a law that would allow gambling on sports in the state.

In papers filed in a New Jersey federal court on Tuesday, the U.S. Department of Justice said it wants to defend the constitutionality of a federal law that restricts sports gambling.

New Jersey's law, signed last year by Governor Chris Christie, would allow sports betting at the state's racetracks and at Atlantic City casinos. It would allow the racetracks and casinos to apply for licenses and open gambling operations for amateur and professional sports.

A slew of leagues, including the National Collegiate Athletic Association, the National Football League, Major League Baseball, the National Basketball Association and the National Hockey League, sued the state in August, saying the law would violate the federal restrictions on sports betting.

The law would "irreparably harm amateur and professional sports by fostering suspicion that individual plays and final scores of games may have been influenced by factors other than honest athletic competition," the leagues said.

Last month, Judge Michael Shipp denied an effort by New Jersey to have the case thrown out, ruling that the leagues had standing to sue.

On Tuesday, the judge granted the government's request to intervene in the case, and said it could participate in oral arguments slated for Feb. 14 on the constitutionality of the federal sports betting regulations.

The case is NCAA et al. v. Chris Christie et al., U.S. District Court, District of New Jersey, No. 12-4947.

For the plaintiff sports leagues: Jeff Mishkin, Anthony Dreyer and Karen Hoffman Lent of Skadden, Arps, Slate, Meagher & Flom.

Follow us on Twitter @ReutersLegal | Like us on Facebook