WASHINGTON — Attorney General Eric Holder Jr. on Thursday will urge reduced sentences for defendants in most of the nation’s drug cases, part of his effort to cut the burgeoning U.S. prison population and reserve stiff penalties for the most violent traffickers.

Holder’s proposal, which is expected to be approved by the independent agency that sets sentencing policies for federal judges, would affect 70 percent of drug offenders in the criminal justice system, according to figures provided by Justice Department officials. It would reduce sentences by an average of nearly a year.

“Certain types of cases result in too many Americans going to prison for far too long, and at times for no truly good public safety reason,” Holder plans to tell the U.S. Sentencing Commission, according to excerpts of his testimony provided to The Washington Post. “Although the United States comprises just five percent of the world’s population, we incarcerate almost a quarter of the world’s prisoners.”

Like Holder’s previous criminal justice reforms, the move is likely to be hailed by civil liberties groups and assailed by some lawmakers who think the administration is chipping away at federal policies designed to deter criminals and improve public safety.

The Sentencing Commission, a seven-member panel that has proposed an amendment to federal sentencing guidelines, will vote on it as soon as April. Until then, federal judges must refer to current sentencing guidelines. Holder, however, will instruct his prosecutors in a memo Thursday not to press judges to impose the longer sentences in the current guidelines if attorneys for drug offenders seek shorter sentences for their clients that would be permissible under the new policy.

Under current mandatory minimum guidelines, a drug offender convicted of possessing 500 grams of cocaine or 28 grams of crack would face a term of 60 to 78 months. Holder is proposing that the time in such a case be reduced to 51 to 63 months.

“By reserving the most severe penalties for dangerous and violent drug traffickers, we can better protect public safety, deterrence and rehabilitation while saving billions of dollars and strengthening communities,” Holder plans to say.

Holder’s new sentencing proposal is the latest step in his agenda to revise the criminal justice system. In August, he announced that low-level nonviolent drug offenders with no connection to gangs or large-scale drug organizations would not automatically be charged with offenses that call for severe mandatory sentences.