WASHINGTON — Tens of thousands of prisoners serving time for federal drug offenses will be eligible to seek early release beginning next year.

The United States Sentencing Commission, which voted in April to reduce the penalties for most drug crimes, voted unanimously on Friday to make that change retroactive. It will apply to nearly 50,000 federal inmates who are serving time under the old rules.

The Sentencing Commission said the move would help ease prison overcrowding and reduce prison spending, which makes up about a third of the Justice Department’s budget. The change comes amid a bipartisan effort to roll back the harshest penalties set during the height of the drug war.

Civil rights groups and prison-reform advocates cheered the decision. Marc Mauer, executive director of the Sentencing Project, called it a “historic shift in the decades-long war on drugs, which has filled half of federal prison cells with people convicted of drug offenses.”