The Supreme Court ruled 5 to 4 yesterday that the Constitution places strict limits on police powers to obtain incriminating statements made by a suspect once formal charges have been filed against him.

The court said that police in Belfast, Maine, acted improperly when they sent an informant, wired with recording equipment, to converse with a defendant awaiting trial for theft and then used the incriminating statements at the man's trial.

Justice William J. Brennan Jr., citing a 1964 ruling, said that police may not "deliberately elicit," through undercover techniques or informants, incriminating statements after someone has been formally charged with a crime. To do so, the court said, would violate a suspect's rights under the Sixth Amendment to have a lawyer present when being interrogated.

Brennan said such statements cannot be used even when a defendant, not the police, initiates the conversation, and even when the police did not seek to obtain information about the crimes for which a defendant had been charged.

But Brennan said prosecutors may nevertheless use any statement a defendant makes about future crimes or other crimes for which he has not been charged. To rule otherwise, he said, "would unnecessarily frustrate the public's interest in the investigation of criminal activities."

Chief Justice Warren E. Burger, in a sharply worded dissent, said the ruling "turns the Sixth Amendment on its head."

"Nothing whatever in the Constitution or our prior opinions supports this bizarre result," Burger said. He said the ruling creates a new right for people who persist in criminal activity even after being indicted for other crimes.

The case, Maine v. Moulton, involves two alleged car thieves in Belfast. The two men were indicted for theft and pleaded not guilty, but one of them, Gary Colson, later confessed and implicated his partner, Perley Moulton. Colson also told police that Moulton was thinking of killing a witness in the case.

The Belfast police chief wired Colson to record a conversation with Moulton, a conversation Moulton thought was going to be about their joint defense strategy. During the conversation, Moulton made some incriminating statements about the thefts and discussed a cover-up.

Moulton, who was never charged with attempting to kill a witness, argued that the statements should be suppressed. A trial judge refused to do so and Moulton was convicted of the charges. But the Maine Supreme Court unanimously reversed that conviction in August 1984, saying the statements should not have been admitted. Moulton, in the meantime, had pleaded guilty to other crimes based on information provided in his conversations with Colson.

Burger, joined by Justices Byron R. White, William H. Rehnquist and Sandra Day O'Connor, said the evidence should not be excluded even if Belfast police violated Moulton's constitutional rights.

Citing recent rulings in which the court has narrowed instances where evidence must be excluded from trial, Burger said that "application of the exclusionary rule to cases like this one deflects the truth-finding process, often frees the guilty, and may well generate disrespect for the law and the administration of justice."

In a second opinion yesterday, the court ruled 6 to 3 that prison disciplinary committees may be sued for violating inmates' rights.

Justice Harry A. Blackmun rejected Justice Department arguments in Cleavinger v. Saxner that prison officials who discipline inmates charged with rules infractions are entitled to absolute immunity of the type enjoyed by judges, prosecutors or witnesses.

The ruling affirms a jury award of $4,500 each to two former federal prisoners in Indiana. Rehnquist, joined by Burger and White, dissented.

In a third case, the court, with Brennan not voting, split evenly, rendering no opinion in Eastern Air Lines v. Mahfoud, which asked whether agreements limiting damages to those injured on international flights also limit any prejudgment interest that might accumulate before the damages are collected.