NEW YORK (AP) _ Convicted killer Robert Chambers forged $7,000 in stolen checks just before his murder trial began and used much of the money to buy cocaine, the prosecutor at the trial has charged.

Chambers' lawyer, Jack Litman, angrily denied the allegations by Assistant District Attorney Linda Fairstein, made in published reports Sunday. Litman said she had made them ''in an obvious effort to save face'' because the trial did not end with a murder verdict.

In a surprise ending to the so-called ''preppie murder'' trial Friday, Chambers pleaded guilty to first-degree manslaughter in the August 1986 death of 18-year-old Jennifer Levin.

The 21-year-old college dropout had maintained throughout the 12-week trial that Miss Levin's death had been an accident, caused when he grabbed her and threw her off him after she hurt him during a sexual encounter in Central Park. But he acknowledged in his guilty plea to manslaughter that he had intended to hurt her.

Ms. Fairstein said Saturday that her office received a tip from an informant that Chambers had stolen the checks in October and used the money for ''drugs and petty cash.'' She did not identify the victim, but the Daily News said court sources described the victim as an elderly woman who possibly was senile.

''Our belief is that he stole and forged $7,000 worth of checks,'' the prosecutor said. ''We knew he was buying drugs, but at first we didn't know where he was getting the money. It went straight up his nose.''

As part of the plea bargain that brought the trial to an end, Chambers pleaded guilty to one unrelated burglary charge, one of three that had been brought against him. Under the bargain, he is immune from prosecution from any other past crimes.

Prosecutors had said that the burglaries were committed to support Chambers' cocaine habit, for which he was institutionalized twice.

Ms. Fairstein said the check theft was ''minor stuff,'' that probably would have ended up as a misdemeanor.

Litman called the allegations ''scandalous.'' He said Ms. Fairstein was upset that Chambers pleaded guilty to manslaughter ''when they proclaimed from the rooftops that he intentionally murdered Jennifer Levin.''

Chambers entered the plea Friday as jurors remained deadlocked on their ninth day of deliberations in the highly publicized case. Both sides feared a hung jury that would have required another emotionally draining trial.

Chambers entered the Rikers Island jail on Saturday to await sentencing April 15. Under the plea bargain, he faces a five-to-15-year prison term.