On the eve of marijuana becoming legal in Washington state, the Justice Department warned that the possession, growing or use of the drug remains illegal under federal law.

“Regardless of any changes in state law, including the change that will go into effect on Dec. 6 in Washington state, growing, selling or possessing any amount of marijuana remains illegal under federal law,” said a statement issued by the U.S. Attorney’s Office in Seattle on Wednesday evening.

Voters in Washington state and Colorado approved ballot initiatives last month that decriminalized the possession of up to an ounce of marijuana by adults. The Washington state law goes into effect Thursday and Colorado’s law will change in the coming weeks.

Authorities in both states have already started dismissing hundreds of misdemeanor cases and retraining state and local police for dealing with the change.

The Justice Department position sets up a potential court fight between the federal government and the states.

Though the measures violated federal drug laws, Justice had refused to provide any guidance despite requests from both states. But the statement made clear that, at least from Justice’s perspective, federal law should prevail. Under federal law, marijuana is considered a Schedule 1 drug, the same category as LSD and heroin.

“The department’s responsibility to enforce the Controlled Substances Act remains unchanged,” the statement said. “Neither states nor the executive branch can nullify a statute passed by Congress.”

The statement warned that it is against federal law to bring any amount of marijuana onto federal property, including national parks and forests.

Justice Department spokeswoman Nanda Chitre declined to comment beyond the statement, which said the department is continuing to review the Colorado and Washington state initiatives.

Sari Horwitz