WASHINGTON – In a ruling that gives new momentum to the national push to legalize marijuana, the U.S. Justice Department said Thursday that it would not interfere with plans by Washington and Colorado to sell and tax pot for recreational use beginning next year.

The department made its long-awaited announcement in a memo to federal prosecutors.

Attorney General Eric Holder had been under growing pressure to respond to the new state laws, since marijuana is still classified as an illegal drug under federal law.

While pot opponents wanted Holder to sue the states to block them from selling a banned substance, the Justice Department said that it won’t bother, as long as the states police themselves well. “Based on assurances that those states will impose an appropriately strict regulatory system, the department has informed the governors of both states that it is deferring its right to challenge their legalization laws at this time,” the department said.

Advocates of legalization cheered the move, calling it a historic step toward ending marijuana prohibition across the United States.

While Washington and Colorado in November were the first to approve marijuana for recreational use, 20 states, including Maine, have approved marijuana sales for medical purposes. Others are expected to vote soon on recreational marijuana, including Alaska in 2014 and California in 2016, lobbyists predict.

Opponents of legalization said the move would lead to a flood of negative consequences.

“We can look forward to more drugged driving accidents, more school dropouts and poorer health outcomes as a new big marijuana industry targeting kids and minorities emerges to fuel the flames,” said Patrick Kennedy, a former Democratic congressman from Rhode Island and co-founder of Project SAM (Smart Approaches to Marijuana), a national alliance that opposes legalization.