Updated at 12:10 p.m. ET

In a memo to staff, Attorney General Jeff Sessions ordered federal prosecutors to "charge and pursue the most serious, readily provable offense" — a move that marks a significant reversal of Obama-era policies on low-level drug crimes.

The two-page memo, which was publicly released Friday, lays out a policy of strict enforcement that rolls back the comparatively lenient stance established by Eric Holder, one of Sessions' predecessors under President Barack Obama.

"This policy affirms our responsibility to enforce the law, is moral and just, and produces consistency. This policy fully utilizes the tools Congress has given us," Sessions told thousands of assistant U.S. attorneys in the memo. "By definition, the most serious offenses are those that carry the most substantial guidelines sentence, including mandatory minimum sentences."

He elaborated on the memo in a brief speech to the Sergeants Benevolent Association of New York City, which honored him with an award Friday in Washington, D.C.

"Charging and sentencing recommendations are bedrock responsibilities of any prosecutor. And I trust our prosecutors in the field to make good judgments," Sessions said. "They deserve to be unhandcuffed and not micromanaged from Washington."

Holder had asked prosecutors to avoid slapping nonviolent drug offenders with crimes that carried mandatory minimum sentences, practices that, as NPR's Tamara Keith explains, "give judges and prosecutors little discretion over the length of a prison term if a suspect is convicted." Holder's recommendation had been aimed partly at helping reduce burgeoning prison populations in the U.S.

Now, if prosecutors wish to pursue lesser charges for these low-level crimes, they will need to obtain approval for the exception from a U.S. attorney, assistant attorney general or another supervisor.

But in his speech Friday, Sessions asserted that the policy change is aimed not at low-level drug users, but rather drug dealers and traffickers.

"If you are a drug trafficker," he said, "we will not look the other way. We will not be willfully blind to your misconduct."

Keith notes this marks a return to the "tough-on-crime philosophy" of the 1980s and '90s — a return that advocacy groups have feared for some time.

"This is a disastrous move that will increase the prison population, exacerbate racial disparities in the criminal justice system, and do nothing to reduce drug use or increase public safety," Michael Collins, deputy director at the Drug Policy Alliance, said in a statement emailed to NPR. "Sessions is taking the country back to the 1980s by escalating the failed policies of the drug war."

The memo also drew a long, scathing rebuke from Holder himself.

"The policy announced today is not tough on crime. It is dumb on crime," he said in a statement. "It is an ideologically motivated, cookie-cutter approach that has only been proven to generate unfairly long sentences that are often applied indiscriminately and do little to achieve long-term public safety."

But Sessions argues the shift in policy is a means of fulfilling the Justice Department's "role in a way that accords with the law, advances public safety and promotes respect for our legal system. It is of the utmost importance to enforce the law fairly and consistently."

And Sessions made it clear that he wants this shift in policy to be immediate.

"Any inconsistent previous policy of the Department of Justice relating to these matters is rescinded, effective today," he wrote.

You can read the full text of Sessions' memo to prosecutors at this link or by scrolling below.

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