A group of 18 Democratic senators charged yesterday that Attorney General William French Smith is violating the law by not appointing a special prosecutor to investigate national security adviser Richard V. Allen's receipt of $1,000 intended for Nancy Reagan.

In a letter circulated by Sen. Thomas F. Eagleton (D-Mo.), the senators criticized Smith's recent statement that the Justice Department would conduct an "extensive investigation" of the matter. That "represents a clear and flagrant violation" of the law, they said, because the department's authority is limited to a very preliminary investigation that "has gone too far already."

A Smith spokesman declined to comment on the letter.

And FBI Director William H. Webster continued to refuse to explain his call to Allen, in which he reportedly told the subject of his agents' investigation that his story had been confirmed by the other participants.

One official close to the case said Smith had approved the contact with the understanding it would be merely a courtesy call to tell Allen the investigation was about to be made public by the Japanese press. Smith wasn't aware until later that Webster also had discussed the substance of the case, the official said.

Sources also said yesterday that the reopened FBI investigation was nearly completed, but Smith isn't expected to make a decision on whether to refer the case to a special prosecutor until after Thanksgiving.

The FBI had finished its initial investigation and Justice prosecutors recommended last week that the case be closed without appointment of a special prosecutor because no evidence of a crime had been uncovered. Sources said Allen and other participants told FBI agents that the money from Japanese journalists was intended as a thank-you gift for Mrs. Reagan for an interview Jan. 21. Allen said he gave the cash to a secretary to turn over to the proper authorities, but that she put the money in a safe and they both forgot about it for eight months.

Justice superiors ordered the FBI to conduct more interviews last week, and officials there said Smith was in no rush to decide whether to ask a special court to appoint an independent prosecutor. He is allowed 90 days, until mid-December, to decide.

Edwin Meese III, the president's counselor, told interviewers Sunday there were no plans to replace Allen. But Meese left open the possibility that Allen will be replaced if a special prosecutor is appointed.

Smith and other Justice officials have urged repeal of the special-prosecutor law, which was enacted in response to the Watergate scandal. But the senators' letter noted Smith's pledge to follow its provisions. The law was enacted "for exactly this kind of case," the letter said. "It is premised on the view that the Justice Department cannot credibly investigate certain high-ranking officials of the administration of which it is a part."

A long preliminary investigation not only "collides" with the intent of the law, "it it positively harmful to Mr. Allen," the senators wrote. The decision to appoint a special prosecutor should be made quickly, "minimizing any adverse inferences against the official in question," they said.

An Eagleton aide said there was no real effort to get Republican senators to sign the letter. "It was a matter of time and a feeling we wouldn't have gotten the Republicans," the aide said. Eagleton circulated the letter at a Democratic caucus last weekend during the long debate over federal spending.

Former attorney general Benjamin R. Civiletti, who twice requested special prosecutors to investigate cocaine-use allegations against White House aides in the Carter administration, said yesterday that he didn't agree that the preliminary investigation had to be as limited as the senators' letter stated.

"My interpretation is that you can interview and re-interview," Civiletti said. "It's not so much the scope of the investigation but its depth, the tools that can be used, such as the grand jury, immunity and plea-bargaining."

In a related development, a spokesman confirmed that Navy Secretary John F. Lehman Jr. sent Allen a statement last week backing Allen's contention that he planned to turn the cash over to authorities. The New York Times reported that Lehman recalled Allen telling him of his "chagrin and amazement" at being handed the money. The spokesman said the secretary wrote the statement Nov. 16 on his own initiative after the story of the investigation was made public.