WASHINGTON (AP) — Despite 75 years of federal marijuana prohibition, the Justice Department said Thursday that states can let people use the drug, license people to grow it and even allow adults to stroll into stores and buy it — as long as the weed is kept away from kids, the black market and federal property.

In a sweeping new policy statement prompted by pot legalisation votes in Washington and Colorado last fall, the department gave the green light to states to adopt tight regulatory schemes to oversee the medical and recreational marijuana industries burgeoning across the country.

The action, welcomed by supporters of legalisation, could set the stage for more states to legalise marijuana. Alaska is scheduled to vote on the question next year, and a few other states plan similar votes in 2016.

The policy change embraces what Justice Department officials called a “trust but verify” approach between the federal government and states that enact recreational drug use.

In a memo to all 94 US attorneys’ offices around the country, Deputy Attorney General James Cole said the federal government expects that states and local governments authorizing “marijuana-related conduct” will implement strong and effective regulatory and enforcement systems that address the threat those state laws could pose to public health and safety.

“If state enforcement efforts are not sufficiently robust … the federal government may seek to challenge the regulatory structure itself,” the memo stated. States must ensure “that they do not undermine federal enforcement priorities,” it added.

The US attorney in Colorado, John Walsh, said he will continue to focus on whether Colorado’s system has the resources and tools necessary to protect key federal public safety interests.

Colorado Governor John Hickenlooper said the state is working to improve education and prevention efforts directed at young people and on enforcement tools to prevent access to marijuana by those under age 21. Colorado also is determined to keep marijuana businesses from being fronts for criminal enterprises or other illegal activity, he said, and the state is committed to preventing the export of marijuana while also enhancing efforts to keep state roads safe from impaired drivers.

Washington Governor Jay Inslee also laid out guidelines for marijuana entrepreneurs.

“If you don’t sell this product to children, if you keep violent crime away from your business, if you pay your taxes and you don’t use this as a front for illicit activity, we’re going to be able to move forward,” Inslee said.

Under the new federal policy, the government’s top investigative priorities range from preventing the distribution of marijuana to minors to preventing sales revenue from going to criminal enterprises, gangs and cartels and preventing the diversion of marijuana outside of states where it is legal.

Other top-priority enforcement areas include stopping state-authorised marijuana activity from being used as a cover for trafficking other illegal drugs and preventing violence and the use of firearms in the cultivation and distribution of marijuana. The top areas also include preventing drugged driving, preventing marijuana cultivation and possession on federal property.

The Justice Department memo says it will take a broad view of the federal priorities. For example, in preventing the distribution of marijuana to minors, enforcement could take place when marijuana trafficking takes place near an area associated with minors, or when marijuana is marketed in an appealing manner to minors or diverted to minors.

Following the votes in Colorado and Washington last year, Attorney General Eric Holder launched a review of marijuana enforcement policy that included an examination of the two states. The issue was whether they should be blocked from operating marijuana markets on the grounds that actively regulating an illegal substance conflicts with federal drug law that bans it.

Peter Bensinger, a former head of the Drug Enforcement Administration, said the conflict between federal and state law is clear and can’t be reconciled. Federal law is paramount, and Holder is “not only abandoning the law, he’s breaking the law. He’s not only shirking his duty, he’s not living up to his oath of office,” Bensinger said.

Last December, President Barack Obama said it doesn’t make sense for the federal government to go after recreational drug users in a state that has legalised marijuana. Last week, the White House said that prosecution of drug traffickers remains an important priority.

A Pew Research Centre poll in March found that 60 per cent of Americans think the federal government shouldn’t enforce federal anti-marijuana laws in states where its use has been approved. Younger people, who tend to vote more Democratic, are especially prone to that view. But opponents are worried these moves will lead to more use by young people. Colorado and Washington were states that helped re-elect Obama.

Advocates of medical marijuana were cautious about the new policy. Twenty states and the District of Columbia have enacted laws that effectively allow patients to access and use medical marijuana. Threats of criminal prosecution and asset forfeiture by US attorneys have closed more than 600 dispensaries in California, Colorado and Washington over the past two years, said Americans for Safe Access, which advocates for safe and legal access to therapeutic cannabis.

Dan Riffle of the Marijuana Policy Project, the nation’s largest marijuana policy organisation, called the policy change “a major and historic step toward ending marijuana prohibition” and “a clear signal that states are free to determine their own policies.”

Kevin Sabet, the director of Project Smart Approaches to Marijuana, an anti-legalisation group, predicted the new Justice Department policy will accelerate a national discussion about legalization because people will see its harms – including more drugged driving and higher high school dropout rates.

Kristi Kelly, a co-founder of three medical marijuana shops near Denver, said the Justice Department’s action is a step in the right direction.

“We’ve been operating in a gray area for a long time. We’re looking for some sort of concrete assurances that this industry is viable,” she said.

A national trade group, the National Cannabis Industry Association, said it hopes steps will be taken to allow marijuana establishments access to banking services. Federally insured banks are barred from taking money from marijuana businesses because the drug is still banned by the federal government.