A White House spokesman said yesterday that President Ford may not be able to introduce legislation proposing statehood for Puerto Rico before he leaves office Jan. 20.

If that is the case, the issue will be left to the incoming administration of President-elect Jimmy Carter, who has said that Puerto Rico, not Washington, should decide the future relationship of the commonwealth to the United States.

That would mean that the question of Puerto Rican statehood could become an inactive issue - at least for the next four years, and possibly for the next eight years.

White House press secretary Ron Nessen said yesterday that the Department of the Interior is now working on legislation proposing Puerto Rico's admission to the union. But he said, "It's a quite complicated subject and it may be that legislation will not be completed in time for this administration to submit it to Congress."

"What will happen then?" Nessen was asked.

"Then it will be up to the next administration to submit it to Congress," he said.

However, H Greg Austin, solicitor for the Interior department, which traditionally prepares such legislation, was more optimistic.

Austin said in an interview following Nessen's daily press briefing that the department has "at least a 60-40 chance" of finishing work on the legislation before Jan. 20.

"I'm betting that we get it done by the 20th," Austin said. "We're in the process of drafting it right now. We're pulling a task force together.

"The statehood act itself is not difficult to write," Austin said." But possible problems could arise over issues like tax preferences for [mainland-based] business in Puerto Rico . . . and offshore [mineral] rights," he said.

Austin cautioned that those "knotty problems" could block completion of the legislation and that Ford may have to leave the matter for Carter after all. But he said he believes that the next President would take a much more positive attitude toward the Ford proposal than reported in the press.

Nessen said President Ford formally will recommend that Congress enact legislation providing statehood for Puerto Rico even if such a bill is not ready for introduction by the time he leaves office.

"It's something that he personally believes in," Nessen said. He added: "If legislation can be completed in time, it will be submitted. Whether the legal document [bill] will be ready to hand over to Congress Jan. 20, I don't know. But the President will stand there [before Congress] and recommend - or he will send in writing a recommendation that Congress pass such legislation."

Nessen said the President's formal recommendation could come in his State of the Union address, which Nessen said would be delivered about Jan. 12.

President Ford proposed statehood for Puerto Rico last Friday while on a skiing vacation in Vail, Colo. Besides Carter's cool response, the proposal also received a lukewarm reception from some members of Congress and from Puerto Rican leaders - including the commonwealth's new governor, Carlos Romero Barcelo.

Although Romero has been known as a statehood advocate, he refused to discuss the issue in his election campaign and did not mention statehood in his inaugural address Sunday.

However, Romero said in an interview with Washington Post Staff Writer Charles A. Krause yesterday that it is 'up to the Puerto RVICAN that it is 'up to the Puerto Rican people voting in plebiscite, to make the final decision" on whether the commonwealth will become a state.

Romero said Fords proposal "demonstrated his interest in defending our right to equal treatment as Americans."

"I reiterate our appreciation of this proposal from the President of the United States," Remero said. "It's effect has been to focus the nation's attention on Puerto Rico . . . An overwhelming majority of the Puerto Rican people want permanent union with the United States and desire to see our bonds . . . strengthened. The President's declaration was made in support of this reality," he said.

And at a press conference yesterday in Puerto Rico, Resident Commissioner-elect Baltasar Corradadel Rio, a political ally of Romero, said that he and the new governor had pressed for the delay of any White House recommendation for statehood.

"In an election year, 1976, it wasn't the best time for Congress to act on puerto Rico's statehood," Corrada said. "We [he and Romero] simply wanted to be in power when these matters were decided, rather than out of power," he said.