The White House now believe the "most likely possibility" is that Jimmy Carter himself asked national securtity adviser Zbigniew Brzezinski to see if Billy Carter could be used as an intermediary with the Libyan government, according to informed sources.

The president took this step at the suggestion of his wife who, sources said, probably posed the idea while the couple was together at Camp David on Nov. 18, 1979.

Rosalynn Carter has recalled suggesting at the time contacting the president's brother to see "if there is anything Billy can do to help" gain the release of the American hostages being held in Iran since last Nov. 4. The president passed "the general idea" on to Brzezinski sources said. probably two days later when Carter returned to Washington from Camp David. p

Brzezinski then phoned Billy Carter three times on Nov. 20, according to these sources. The result was the now-controversial meeting of Nov. 27 at which Billy Carter introduced the Libyan charge d'affaires, Ali Houderi, to the national security adviser.

President Carter, his wife and foreign policy adviser may all be called to testify in person on these events before the special Senate subcommittee said yesterday they want the chief executive to appear in person before them for questioning. [Details on Page A10].

The details of how Billy Carter acted as international go-between for the Libyan government are in controversy because he was under investigation last fall by the Justice Department for failure to register as an agent of a foreign government. Last week, after many months of investigation, Billy Carter conceded that he was -- and filed the required statement.

From the details revealed thus far, one issue is whether the Carter administration -- and now the president himself -- helped to enhance Billy Carter's stature and value to the Libyan government by using him as an intercessor.

The Carter White House has said repeatedly that its only interest in contacting Billy about Libya was to search for any diplomatic channel available that might help win release of the American hostages.

In any case, less than two months after the Nov. 27 meeting and a subsequent session between Houderi and President Carter, Billy Carter received his first check from the Libyan government -- $20,000 delivered from the same Libyan diplomat whom he took to the White House. In all, the president's brother received $220,000 from the Libyans, which the Justice Department concluded was payment for his work as a foreign agent.

White House sources yesterday maintained, however, that the president made the suggestion to Brzezinski on Nov. 20 with no thought of such implications. The president's thoughts were totally preoccupied with seeking ways to solve the Iranian hostage crisis, the sources recalled. On the same day the president supposedly passed the suggestion to Brzezinski, he was issuing a public statement that warned that the United States might use arms against Iran to free the Americans.

This new disclosure, involving the president, is at least the third different version offered this week of how it happened that Billy Carter wound up at a White House diplomatic meeting.

White House press secretary Jody Powell said yesterday he and White House counsel Lloyd Cutler are still trying to piece together exactly how Billy Carter was first approached last fall. At yesterday's press briefing, Powell said he will try to give to more detailed account soon, possibly today.

When the controversy began this week, Powell maintained that he was not sure if the president ever knew of the Brzezinski-Houderi-Billy Carter meeting when it occured.

Yesterday, however, Powell said the president's records show he had approved it beforehand -- a fact that made "more likely," sources said, that the president had actually passed the idea on to Brzezinski originally.

One confusing part of this newest version of these events is the role of Rosalynn Carter. Yesterday, Powell said Mrs. Carter remembered calling Billy Carter herself from Camp David and asking him if he thought there was anything he could do.

Powell added, however, that Rosalynn Carter thought her call had taken place after Brzezinski had already called Billy Carter. Thus, it seemed unlikely that Mrs. Carter initiated the idea.

Yesterday, however, sources said Mrs. Carter, "most likely mentioned Billy to the president" over that Camp David weekend. She may have called the president's brother at that time or later, as she still believes.

In any case, it is now virtually assumed by the White House that the president passed the suggestion on to his adviser or Brzezinski took it from there.

The first version of events was provided by the White House Tuesday with the so-called "white paper," which said that contacting Billy Carter was Brzezinski's idea. White House aides emphasized that it was his, alone. There was no mention at the time of any involvement by either the president or Mrs. Carter.

On Wednesday and again Thursday, Powell and other White House aides began saying publicly that they were not certain where Brzezinski got the idea to call Billy Carter.

At that point, Powell added Mrs. Carter's name to those who might have suggested to Brzezinski that Billy Carter be called.

The president, however, was never mentioned as a possibility. On Thursday, Powell told reporters at his briefing "the president is not sure" whether he knew ahead of time of the Nov. 27 meeting.

Yesterday, according to sources, a search of the president's personal records revealed that he definitely knew about the meeting ahead of time. And that discovery led to the conclusion that it was most likely he who set the chain of events in motion.