PORTLAND, Ore. — The U.S. Postal Service says it’s illegal to mail materials containing advertising for marijuana products, even in states that have legalized the federally controlled substance.

The policy statement released this week comes in response to a letter from Oregon’s Congressional delegation asking the service to clarify its policy on the issue.

In November, a memo distributed in the Portland postal district said it was unlawful for newspaper outlets to run marijuana ads and use the U.S. mail for delivery.

The memo caused confusion among publishers whose newspapers have published ads for dispensaries and manufacturers in the region.

“Advertisements for the sale of marijuana are non-mailable,” Thomas Marshall, executive vice president and general counsel of the Postal Service, wrote in a letter to the delegation. That’s because under the federal Controlled Substances Act marijuana’s sale is prohibited, he said. That same law also prohibits placing written ads for controlled substances like marijuana in newspapers, magazines or other publications.

“These provisions express Congress’s judgment that the mail should not be used as a means of transmitting advertisements for the sale of marijuana, even if that sale is allowed under state law,” Marshall wrote.

Marshall says the Postal Service has released a national policy, which also spells out that local postal officials can’t refuse mail that contains pot ads, but they must report it; the matter must then be turned over to law enforcement agencies who can decide if an investigation is warranted.

In a joint statement, Reps. Earl Blumenauer and Suzanne Bonamici and Sens. Ron Wyden and Jeff Merkley called the agency’s stance uncompromising and said they want federal authorities to respect decisions made by Oregonians, who last year voted to legalize recreational marijuana for adult use. Oregon voters legalized medical marijuana in 1998.

Medical marijuana dispensaries have multiplied in Oregon and many now also offer recreational pot. The industry uses billboards, websites and newspaper ads to showcase their products in a highly competitive market.

“Unfortunately,” said the joint statement, “the outdated federal approach to marijuana as described in the response from the Postal Service undermines and threatens news publications that choose to accept advertising from legal marijuana businesses in Oregon and other states where voters also have freely decided to legalize marijuana.”

The Oregon Newspaper Publishers Association said the Postal Service policy could rob newspapers of revenue. The group has advised its members not to run marijuana ads if they use the postal service for some delivery.

“This national policy from the USPS definitely prevents some of our members advertising opportunities,” said executive director Laurie Hieb. But, she added, “This is new advertising and none of them currently rely on it to stay in business.”

Some outlets — like two dozen newspapers in the Pamplin Media Group — have already decided not to run pot ads, but for a completely different reason: because they don’t promote health.

In addition to Oregon, the policy also impacts the states of Washington, Colorado and Alaska, where voters legalized recreational marijuana in recent years, as well as other states where medical marijuana is legal.