Attorney general Jeff Sessions is directing federal prosecutors to pursue the most serious charges possible against the vast majority of suspects, a reversal of Obama-era policies that is sure to send more people to prison and for much longer terms.

The move has long been expected from Sessions, a former federal prosecutor who cut his teeth at the height of the crack cocaine epidemic and who has promised to make combating violence and drugs the justice department’s top priority.

“This policy affirms our responsibility to enforce the law, is moral and just, and produces consistency,” Sessions wrote in a memo sent Thursday night to US attorneys and made public early on Friday.

The move amounts to an unmistakable undoing of Obama administration criminal justice policies that aimed to ease overcrowding in federal prisons and contributed to a national rethinking of how drug criminals were prosecuted and sentenced. Critics said the change will subject more lower-level offenders to unfairly harsh mandatory minimum sentences.

Sessions contends a spike in violence in some big cities and the nation’s opioid epidemic show the need for a return to tougher tactics.

“The opioid and heroin epidemic is a contributor to the recent surge of violent crime in America,” Sessions said in remarks prepared for a Thursday speech in Charleston, West Virginia. “Drug trafficking is an inherently violent business. If you want to collect a drug debt, you can’t, and don’t, file a lawsuit in court. You collect it by the barrel of a gun.”

The policy memo says prosecutors should “charge and pursue the most serious, readily provable offense” – something more likely to trigger mandatory minimum sentences. Those rules limit a judge’s discretion and are typically dictated, for example, by the quantity of drugs involved in a crime.

Jeff Sessions visits a border fencer in San Diego, California, on 21 April. Photograph: Mike Blake/Reuters

The memo concedes there will be cases in which “good judgment” will warrant a prosecutor to veer from that rule. But any exceptions will need to be approved by top supervisors, and the reasons must be documented, allowing the justice department to track the handling of such cases by its 94 US attorney’s offices.

Even if they opt not to pursue the most serious charges, prosecutors are still required to provide judges with all the details of a case when defendants are sentenced, which could lengthen prison terms.

The requirements “place great confidence in our prosecutors and supervisors to apply them in a thoughtful and disciplined manner, with the goal of achieving just and consistent results in federal cases”, the memo states.

The directive rescinds guidance by Sessions’ Democratic predecessor, Eric Holder, who told prosecutors they could in some cases leave drug quantities out of charging documents so as not to trigger long sentences. Holder’s 2013 initiative, known as Smart on Crime, was aimed at encouraging shorter sentences for nonviolent drug offenders and preserving justice department resources for more serious and violent criminals.

Though Holder did say that prosecutors ordinarily should charge the most serious offense, he instructed them to do an “individualized assessment” of the defendant’s conduct. And he outlined exceptions for not pursuing mandatory minimum sentences, including if a defendant’s crime does not involve violence or if the person doesn’t have a leadership role in a criminal organization.

The Obama policy shift coincided with US Sentencing Commission changes that made tens of thousands of federal drug prisoners eligible for early release, and an Obama administration clemency initiative that freed convicts deemed deserving of a second chance.

Combined, those changes led to a steep decline in a federal prison population that now stands at just under 190,000, down from nearly 220,000 in 2013. Nearly half of those inmates are in custody for drug crimes, records show.

Obama administration officials cited that decline and a drop in the overall number of drug prosecutions as evidence that policies were working as intended. They argued prosecutors were getting pickier about the cases they were bringing and were seeking mandatory minimum sentences less often.

Still, some prosecutors felt constrained by the Holder directive and expressed concern that they’d lose plea bargaining leverage – and a key inducement for cooperation – without the ability to more freely pursue harsher punishments.

Sessions and other justice department officials argue Holder’s approach sidestepped federal laws that impose such sentences and created inconsistency across the country in the way defendants are punished. Even while in the Senate, Sessions repeatedly asserted that eliminating mandatory minimums weakened the ability of law enforcement to protect the public.

Advocates for the previous policy said the change will revive the worst aspects of the drug war.

“It looks like we’re going to fill the prisons back up after finally getting the federal prison population down,” said Kevin Ring, president of Families Against Mandatory Minimums. “But the social and human costs will be much higher.”