Preppie killer Robert Chambers, who served 15 years behind bars for strangling a woman in Central Park during what he said was rough sex, is headed back to prison for a drug offense.

Chambers pleaded guilty yesterday to criminal sale of a controlled substance and assault on a police officer. He and his girlfriend were arrested in October 2007 in an undercover sting at their Manhattan apartment on charges of dealing cocaine.

The district attorney’s office said Chambers has been promised 19 years and four months in prison when he is sentenced September 2. Chambers will get six years on the assault charge, which will run concurrently, and will have five years of supervision after he’s released.

A telephone call to Chambers’s defense attorney, Valerie Van Leer-Greenberg, was not immediately returned yesterday.

Chambers initially pleaded not guilty to the charges and was facing life in prison if convicted after trial. His attorney had planned to present a psychiatric defense at trial, arguing his brain and judgment were damaged because of years of drug abuse. She had said he was using up to 12 bags of heroin plus other drugs each day at the time of his arrest.

Chambers’s girlfriend, Shawn Kovell, admitted in state Supreme Court in December that the two sold narcotics to an undercover detective. Under the terms of her plea deal, she was sent to a drug rehabilitation center, and upon completion of her program she will be allowed to withdraw the guilty plea and plead to a lesser charge so she can receive a sentence of probation.

Her lawyer, Frank Rothman, said at the time she was a drug addict and it was her first arrest.

Chambers, now 41, became tabloid fodder in 1986 after the death of Jennifer Levin, an 18-year-old graduate of the exclusive Baldwin School, during a tryst in Central Park.