Sessions told Justice Department attorneys they should apply the most serious charge possible that can be proven in court.
In a two-page memo, Sessions told Justice Department attorneys around the country that, when prosecuting a case, they should apply the most serious charge possible that can be proven in court.
"By definition, the most serious offenses are those that carry the most substantial guidelines sentence, including mandatory minimum sentences," the memo said.
Sessions said the new stance "advances public policy and promotes respect for our legal system."
It comes as the new administration of President Donald Trump has promised a crackdown on violent crime, illegal immigration and drug trafficking.
It could reverse the directives of former president Barack Obama's attorney general Eric Holder, who -- addressing a three-decade old swelling of the prison population with largely black men -- sought to end the widespread application of harsh "mandatory minimum" sentences for relatively small and non-violent crimes.
Sessions' order said prosecutors in most cases should stick to recommending sentences within the statutory rules, following the mandatory minimums.
Prosecutors wanting to recommend lighter punishment than the official standard, Sessions said, will have to get permission from their superiors, in addition to providing written justification.
Under Holder, US attorneys were allowed to negotiate lesser charges and lighter sentences to avoid, for example, a second or third-time non-violent drug offender from being jailed for decades under mandatory minimum sentencing laws.