WASHINGTON – Federal courts may be asked to review tens of thousands of criminal sentences in light of a Supreme Court ruling yesterday that says judges are no longer bound by federal sentencing guidelines.

The ruling vastly expands the discretion of judges to set sentences, within minimum and maximum limits proscribed by law.

A 5-4 majority of the court relegated the hitherto mandatory guidelines to “advisory” status, reversing a judicial and legislative trend of judges playing a diminishing role in setting sentences.

How individual sentencing judges and appellate courts will apply their new freedom remains unclear. Lawyers and law professors predicted large variations from judge to judge and one region to another.

“This is the most amount of judicial discretion ever afforded to sentencing judges. … Sentences are going to be all over the lot,” said a former federal prosecutor and law professor at the Indiana University School of Law, Frank Bowman.

The change in ground rules also offers convicts an opportunity to challenge their sentences.

“There are 170,000 federal inmates right now.There are lots more on probation and lots more facing charges. There are hundreds of thousands of people who can walk into court with a nonfrivolous assertion that this changes their universe,” said a law professor at Ohio State University, Douglas Berman.

But defense lawyers cautioned that while some judges have expressed frustration with the guidelines, many would likely uphold previous sentences.

“I don’t think the prison cells are going to be empty after today,” said the vice president of the National Association of Criminal Defense Lawyers, Carmen Hernandez.

The Bush administration expressed disappointment with the decision and announced that it would continue to push judges to apply sentences according to the guidelines.

“To the extent that the guidelines are now advisory,the risk increases that sentences across the country will become wildly inconsistent,” said the assistant attorney general, Christopher Wray.

Mr. Wray credited the sentencing guidelines with reducing crime and taking violent criminals off the street. Congressional power to rewrite the law was acknowledged by the court, and Mr. Wray said he would work with lawmakers on the issue.

“Ours, of course, is not the last word: the ball now lies in Congress’ court,” Justice Breyer wrote.

The chairman of the Senate Judiciary Committee, Senator Specter of Pennsylvania, said he would review the ruling, saying it has “major implications for America’s legal system.”

“I intend to thoroughly review the Supreme Court’s decision and work to establish a sentencing method that will be appropriately tough on career criminals, fair, and consistent with constitutional requirements,” Mr. Specter said in a statement.

The decision had been widely anticipated by lawyers and judges after an earlier ruling in a case known as Blakeley sowed confusion in federal sentencing by striking down a Washington State scheme that was similar to the federal guidelines. But instead of greater clarity, lawyers and law professors said yesterday the court had created new “chaos.”

Justice Scalia wrote in a dissent that the majority opinion will “wreak havoc on federal district and appellate courts quite needlessly, and for the indefinite future.”

The new rules could also have the effect of reducing the leverage of the government in plea bargains, especially for some white-collar crimes, where there are no statutory maximums.

“What bargaining leverage the government has is certainly going to be diminished,” Mr. Bowman said.

The court’s opinion was fractured into two separate 5-4 decisions that appeared to be the result of a power struggle on the court over the relative roles of judges and juries in sentencing.

The first part of the decision held that judges cannot apply mandatory guidelines to increase sentences based on facts that have not been proven to a jury beyond a reasonable doubt. Mandatory increases based on judge found facts violate the Sixth Amendment’s guarantee of a trial by jury, the court ruled in an opinion written by Justice Stevens and joined by Justices Scalia, Souter, Thomas, and Ginsburg.

That opinion effectively reduced the power of judges to set sentences.

But a second portion of the opinion, which addressed the remedy for the constitutional violation, was authored by a different group of judges.

Written by Justice Breyer and joined by Justices Kennedy, O’Connor, Rehnquist, and Ginsburg, the remedy dramatically increased the discretion of judges by making the guidelines advisory.

Mr. Bowman said Justice Breyer had pulled off “a remarkable act of judicial jujitsu” by turning a decision about the right to a jury trial into a vehicle that expanded the discretion of judges.

The dissenting judges wrote that they would have kept the guidelines mandatory but imposed a requirement that the facts to which they applied be found by a jury.

The decision represents “almost a complete u-turn” in the court’s recent sentencing jurisprudence, which had narrowed the power of judges to influence sentences, Mr. Bowman said. “We are [now] on the opposite train,” he said.

While defendants won a constitutional victory for their jury trial rights, they lost in the way it will be applied.

“This is bittersweet for the defendants, because the opinion recognizes Sixth Amendment rights, but then makes one person responsible for increasing sentences using facts not found by a jury,” said an Arizona federal public defender and chairman of the Federal Defender Guideline Committee, Jon Sands.

According to the ruling, district judges will no longer be told they “shall impose a sentence …within the range” established by the guidelines. Instead, they will need only to “consider” that range as one of many factors, including the need for punishment, deterrence, and protecting the public.

The majority also gave the appellate courts power to review the sentences for “reasonableness,” which will presumably lead to additional appeals.

Justice Breyer reasoned that Congress would have preferred the removal of the mandatory language rather than the invalidation of the entire law or a declaration that all facts had to be proven by jury.

The dissenting judges countered that Congress had created the guidelines precisely to constrain judicial discretion and to ensure more consistent sentences across the country.

The court-imposed overhaul “turns the entire system on its head” and “runs contrary to the central purpose that motivated Congress to act in the first place,” Justice Stevens wrote.

Justice Scalia warned that the decision will lead to unpredictable results.

“The worst feature of the scheme is that no one knows – and perhaps no one is meant to know – how advisory guidelines and ‘unreasonableness’ review will function in practice,” he wrote.

Critics of the mandatory guidelines praised the decision, saying its will shift power from prosecutors to judges.

“The net effect will be an improvement in the administration of justice, because we are more likely to find wisdom and prudence from impartial judges than from partial prosecutors,” said the director of the Cato Institute’s Project on Criminal Justice, Tim Lynch.

The ruling arose out of two cases in which judges vastly increased the sentences based on facts not proven at trial. In one case, a jury convicted a Wisconsin man, Freddie Booker, of possession of 50 grams of crack cocaine, and sentenced him to 23 years and 10 months in prison. At a sentencing hearing, the judge found that he had actually possessed an additional 566 grams and sentenced him to 30 years, in accordance with the guidelines.

Yesterday, the high court remanded his sentence for reconsideration.

A lawyer for Mr. Booker, Dean Strang, said yesterday that his client may still wind up with the same sentence he had under the previous system.

In the second case, a judge found additional facts that would have added 10 years to the 5- to 6-year sentence authorized by the jury for a Massachusetts man, Ducan Fanfan, convicted of cocaine trafficking. However, the judge declined to impose the longer sentence, citing the court’s earlier decision to strike down the Washington sentencing guidelines. His case was also remanded for resentencing.