A divided U.S. Court of Appeals here warned federal district judges yesterday that they must not allow admissions of a defendant's guilt into evidence in criminal trials when the admissions were obtained by police as part of a promise of leniency to a suspect.

The 2-to-1 ruling came in a case involving Kim L. Powe, who was convicted in 1976 of illegally distributing one tablet of the drug Phenmetrazine to an undercover D.C. policeman. She was sentenced to serve up to three years in prison and already has been released from the Federal Correctional Institute in Lexington, Ky.

The dispute arose over the testimony of D.C. police detective Rosewal Yates, who told the jury at Powe's trial that he asked her if "she wanted to cooperate and to assist herself in this case and help herself out."

Shortly after that, Yates testified, Powe admitted that she had participated in the drug sale to the undercover agent, Gregory Green.

Circuit Judge David L. Bazelon wrote in the opinion, concurred in by Judge Edward A. Tamm, that the appellate court "has recognised (that) coerced statements are inherently suspect, and the methods of coercion are not limited to acts of physical brutality."

The majority criticized U.S. District Court Judge Thomas A. Flannery for not conducting an inquiry into how voluntary Powe's admission was before allowing Yates' testimony. The majority also critized Powe's court-appointed lawyer, James G. Heldman, as being "ill-prepared" for not raising the issue of the voluntariness of Powe's admissions. The issue was raised by another court-appointed attorney in the appeals court.

Heldman said he would not comment on the ruling until he had a chance to read the opinion.

The appeals court did not order a new trial for Powe, but said Flannery should conduct a hearing on the voluntariness of Powe's statements.