The principal backer of Washington's new mandatory minimum sentencing law will ask the City Council today to delay implementation of the measure for 90 days, following pleas from prosecutors for more time to gear up for the new law.

Council member John Ray (D-At large), author of the measure, said yesterday he agreed to ask for an emergency delay after U.S. Attorney Stanley S. Harris said he needs more time before his prosecutors can implement the statute, approved by D.C. voters last fall.

The required period for congressional review of the law expires Thursday, and the measure is scheduled to take effect Friday. Harris said he was not aware the statute would go into effect immediately.

Ray said Harris, who is scheduled to address the council today, "sort of dropped the ball" by allowing problems with the law to go unresolved until just days before it was to take effect, but said that he will propose the delay nevertheless.

A spokeswoman said that Ray is also willing to discuss criticisms of the law voiced by Harris, Public Defender Service director Frank Carter, officials of the D.C. Bar and other lawyers. However, Ray said he is not convinced that any changes in the law are necessary.

"John Ray is not going to let them go out and do away with what we voted for," Ray said, vowing to oppose attempts to make substantial changes in the law.

Council Chairman David Clarke said he will support the 90-day delay and expects it to be approved by the council, adding that "it just gives the U.S. Attorney time to prepare for the effective implementation of the law."

The law, known as Initiative 9 and approved overwhelmingly by voters last September, requires judges to impose mandatory minimum sentences for crimes involving firearms and for many drug-related offenses.

The U.S. Attorney's office, along with a majority of the City Council, D.C. Del. Walter E. Fauntroy and local chapters of the NAACP, Urban League and League of Women Voters, originally opposed the measure as unneccessary said it could create confusion in the court system.

Harris, explaining his request for the delay, yesterday said only that "we're not in a position to implement the law yet." Margaret Gentry, spokeswoman for Ray, said, "We went through all of these arguments last year. John's position hasn't changed. John thinks it's fine the way it is."

As an example of the new law's provisions, it would require a mandatory five years for a first conviction of an armed offense and a mandatory 10 years for a second offense.

The law also requires a minimum of four years in jail for persons convicted of selling narcotic drugs such as heroin and 20 months for selling nonnarcotic drugs such as phencyclidine or cocaine.

A subcommittee of the bar recently concluded that the new mandatory sentences for armed offenses would sharply reduce plea-bargaining and send more cases to trial. Prosecutors, the bar said, would be reluctant to bargain down to more lenient, unarmed offenses, while defendants would rather face a jury than plead to the new minimum sentences.

The bar objected to the sentences for drug-related offenses because the law makes an exception for drug addicts. Almost every defendant, the bar said, could claim addiction. Attorneys contend that numerous "minitrials" would be necessary to decide the addiction issue at sentencing.