The Justice Department yesterday said it plans to implement a major new parole policy for federal prisoners that will force serious offenders--including convicted murderers, airplane hijackers, and kidnapers among others--to serve longer prison terms.

The changes, contained in guidelines of the U.S. Parole Commission that will become effective next Monday, provide that offenders in the most serious categories who have long criminal histories would have to serve 15 years of a sentence before becoming eligible for parole. Previously, such offenders would have been eligible after eight years and four months.

A serious offender without a lengthy criminal record would be eligible for parole after eight years and four months, almost twice the previous time of four years and four months.

Parole commission chairman Benjamin F. Baer said, "The impact of these changes will be to increase the time customarily served by the most serious offenders, such as those incarcerated for murder, aircraft piracy, large-scale sale of drugs and offenses involving particularly dangerous weapons. This has been done to further promote respect for the law and to protect society by isolating the serious offender."

The guidelines apply only to federal prisoners, of which there are 29,403 in the U.S. prisons. How many of them have been convicted of the most serious categories of crimes could not immediately be determined yesterday.

Crimes that would fall into the "most serious" category include murder, terrorism, espionage, airplane hijacking, kidnaping, drug sales of three or more kilograms [6.6 pounds] of heroin, and forcible crimes that result in death, including rape, robbery and arson.

The guidelines also provide increased mandatory prison time for large sales of marijuana and crimes that involve weapons, including sawed-off shotguns, machine guns or silencers.

For adult inmates convicted of selling 10 tons or more of marijuana, the minimum time for offenders with serious criminal pasts was increased from five years to 6 1/2 years. The mimimum parole time for a person considered to be a slight criminal risk was increased from two years to three years and four months.

Persons with long criminal records convicted of crimes involving a sawed-off shotgun or a machine gun will have a minimum parole time of five years compared to two years and 10 months

For less serious crimes, the parole guidelines will remain about the same.

Peter Hoffman of the U.S. Parole Commission said that under current law, federal judges are not bound by the parole commission guidelines but generally have abided by them. Once a prisoner is sentenced, it is up to the parole commission to determine when he or she will be eligible for parole within the sentence.