SAN FRANCISCO (Reuters) -- A U.S. district judge Wednesday issued an order temporarily barring the federal government from prosecuting California doctors who recommend marijuana to their patients.

Judge Fern Smith of San Francisco issued the preliminary injunction after settlement negotiations broke down between federal authorities and a group of doctors and patients who advocate marijuana for medical use.

"This is a major victory for the doctors and the patients," said attorney Graham Boyd, who represents the doctors and patients who sued the federal government.

The U.S. Department of Justice said it may appeal. "We're carefully reviewing Judge Smith's decision and we will consider all of the legal options," department spokesman Gregory King said.

A group of California doctors, patients, medical associations and the American Civil Liberties Union (ACLU) filed suit in January, seeking a court order blocking the federal government from acting against doctors who recommend marijuana under a new state law.

California voters last November approved Proposition 215, which makes it legal for sick people in the state to grow and possess marijuana for medical use when recommended by a doctor.

Supporters of the controversial measure say marijuana can help ease the pain of AIDS, cancer, glaucoma and other serious illnesses.

The Clinton administration maintains that marijuana is illegal under federal law and has pledged to punish doctors who recommend its use.

Supporters of the marijuana measure said the government's threat discourages doctors from talking to their patients about marijuana, violating their free speech rights.

Government attorneys told the court that the Clinton administration's stance did not prevent doctors from discussing the benefits and possible risks of marijuana, only from recommending or prescribing it.

Smith issued a temporary restraining order two weeks ago, blocking the federal government from prosecuting doctors until May 1, and ordered both sides to negotiate a settlement.

The federal government and the doctors and patients held several negotiating sessions to try to settle the dispute.

"The negotiations fell apart and we concluded we couldn't settle," Boyd said.

In response, Smith on Wednesday issued the preliminary injunction, which shields doctors treating patients with AIDS, cancer, glaucoma and debilitating illnesses that cause seizures or muscle spasms.

Smith said the injunction would limit "the government's ability to prosecute physicians, revoke their prescription licenses, or bar their participation in Medicare and Medicaid because they recommend medical use of marijuana."

Boyd said the preliminary injunction would shield from prosecution "virtually every" doctor that would recommend marijuana.

Unless overturned on appeal, the preliminary injunction would be in place until the legal dispute works its way through the courts, a process that could take several years, Boyd said.