An advisory committee appointed last year to help implement the District of Columbia's no-fault auto insurance law voted yesterday to oppose Mayor Marion Barry's efforts to delay the law's taking effect from April 1 until October.

Critics of the mayor's effort said they feared a delay in implementation of the controversial measure would give opponents of no-fault more time to try to scuttle it. The no-fault bill narrowly passed the City Council, and Barry allowed it to become law without his signature.

"The real issue is that if this law does not go into effect April 1, it may never go into effect," said attorney Robert Joost, a member of the No-Fault Insurance Steering Committee appointed by James Montgomery, acting superintendent of insurance for the District.

The committee, made up of insurance, consumer and city representatives, voted 6 to 2, with 2 abstensions, to urge Montgomery to oppose the delay at a City Council hearing on Barry's proposal scheduled for today.

Montgomery could not be reached for comment.

Barry has said he favors no-fault insurance, but that he has doubts about the law passed by the council. Despite those reservations, Barry has denied any attempt to kill the legislation.

One major reason Barry cited for the proposed delay was a potential increase of 32 percent in some insurance rates. However, according to the insurance department, that large increase would affect only the category of drivers who are the highest risk and reflects other factors in addition to no-fault. Other increases are much lower and directly related to no-fault.

Meanwhile yesterday, the only city actuary working on the no-fault law announced he was leaving the government March 1 to join Amalgamated Casualty, a local auto insurance company.

The actuary, Fred L. Brewer, a city employe since 1975, has been reviewing rate schedules and regulations to implement no-fault. He said he had been negotiating with Amalgamated for several months and downplayed reports he was frustrated with the slow pace of implementing the no-fault law.

"When you are trying to get things done, you have to speak loudly. You try to get things moving," said Brewer, who returned a reporter's telephone calls only after he had spoken with Barry.

"He got an offer he couldn't refuse," Barry said, referring to Brewer's new job.

The no-fault law was the subject of months of bitter political maneuvering. Insurance companies, which could lower their costs of underwriting policies, lobbied the council and the mayor strenuously for passage of no-fault. Trial lawyers, who would lose business as a result of the law, lobbied against it.

As the situation now stands, the law would take effect April 1. Thereafter, motorists renewing their auto tags would have to show proof of insurance.