Washington ? The Supreme Court abandoned nearly two decades of federal sentencing practice Wednesday, saying judges no longer have to follow the complex system of guidelines that Congress designed in the 1980s to make jail terms tougher and more uniform.

In a 5-4 decision, Justice John Paul Stevens, writing for the majority, found that the federal guidelines ran afoul of the court’s 2004 ruling that said the Sixth Amendment required juries, not judges, to determine facts that can lengthen sentences.

But rather than eliminate the guidelines or let lower courts work out how to apply the 2004 ruling within the guidelines the court, in a separate 5-4 decision written by Justice Stephen Breyer, prescribed a sweeping fix of its own.

The guidelines, it said, will be used in an “advisory” manner to help federal judges come up with “reasonable” sentences.

The court said that solved the constitutional problems with the guidelines, and preserved Congress’ intent in adopting the rules. But criminal justice experts said Wednesday that the rulings also could bring more confusion to the process.

“The court, by fiat or constitutional interpretation, has now created a system of advisory guidelines, so what we have now is essentially unconstrained judicial sentencing,” said Frank Bowman, an Indiana University law professor and a former federal prosecutor.

Bowman noted that for years, many federal judges have complained that the guidelines left them too little flexibility in determining sentences, and Wednesday’s ruling might seem to be what they wanted.

“But I think they’ll come to regret what has happened here, because it’s something that’s far more likely to provoke a response from the Department of Justice and Congress that they’ll find unpleasant,” he said.

It’s unclear, Bowman and others said, whether Wednesday’s rulings offer most of the 170,000 current federal inmates a chance to appeal their sentences. But most agree that nearly all inmates are likely to think they have appeals, inspiring thousands to file.

Wednesday’s decisions are the latest in a line of cases in which the court has been reconsidering the constitution’s requirements regarding sentencing.

An unusual alliance of justices, including Stevens, Antonin Scalia, Clarence Thomas, and justices David Souter and Ruth Bader Ginsburg, has led an effort to heighten the standards used at sentencing and require juries to make the most important determinations.

Ginsburg has been viewed as the key vote to that alliance, and Wednesday’s opinion saw her play the most crucial role. She was the only justice who agreed with the Stevens’ assertion that the sentencing guidelines violated the Sixth Amendment and with Breyer’s proposed remedy.

“I think she is the real surprise here, going along with the court’s remedy,” said Jon Sands, a federal public defender. “That’s something that many of us didn’t see coming.”