President Reagan, in a letter to Chancellor Helmut Schmidt of West Germany, has committed the United States to start negotiations with the Soviet Union between mid-November and mid-December on limiting nuclear missiles in Europe.

Reagan was also quoted today as advising Schmidt, who is extremely anxious that negotiations begin soon, not to be misled by published remarks by others in Washington who may create different impressions of U.S. intentions. Reagan called such remarks "pure speculation."

Portions of the letter, which is dated Juyly 17 and reportedly was received by Schmidt just before this week's Ottawa summit, appeared today in five major West German newspapers. Also included in the press reports were comments by an unidentified senior West German governmental source who described Schmidt as attaching great importance to Reagan's statements.

[In Washington, a White House spokesman said, "There is a letter reaffirming the U.S. commitment to talk to the Soviets (on nuclear missiles in Europe) before the end of the year."] He declined to comment on its specific contents.)

Although the quoted parts of the Reagan letter contain little new about the U.S. position, the letter's importance to Schmidt is that he can now use Reagan's unconditional personal guarantee to squelch persistent doubts here about U.S. policy on the European-based missiles.

Skepticism about U.S. promises to pursue serious negotiations with the Soviet Union has fed growing opposition, both in Swchmidt's Social Democratic Party and in West Germany, to the planned stationing in Western Europe of U.S.-made Pershing II and cruise missiles beginning in late 1983.

This skepticism was boosted by what had appeared here to be conflicting signals about the Reagan administration's true intentions. Senior U.S. officicals would assure Western Europe in recent months of America's interest in pursuing negotiations, while press reports in Washington would quote other officials suggesting that the United States had no such immediate aims. Differences in accent on security policy between Secretary of State Alexander M. Haig Jr. and Defense Secretary Caspar W. Weinberger were also cited with concern here.

The senior German government source stressed in today's press reports that in the future Bonn would rely on statements by Reagan and Haig on matters concerning U.S. foreign policy and will regard statements made by others in Washington as "noise."

The 1979 NATO missiles decision, seen by the West as necessary to offset a buildup of similar Soviet weapons, included a NATO offer to negotiate limitations on such weapons with the Soviet Union.

In his letter, Reagan noted that Haig told NATO foreign ministers meeting in Rome last May that the United States wanted to begin negotiations with the Soviet Union by the end of this year. Haig restated the position in his arms control speech in New York on June 14.

"I made this decision personally" on the negotiations, Reagan is quoted as writing Schmidt, according to an unofficial translation of the German text of the letter, "and I would like to take this opportunity to assure you most clearly that I am absolutely committed to following this decision through . . . I am assuming that we will be in a position to begin formal negotiations between mid-November and mid-December. . . . . I am determined to make sure that very thorough consultations take place between us before the negotiations begin."

In a reminder that the new missiles are intended to guard against a Soviet threat to Europe and not to the United States, Reagan wrote: "The fact that the Soviet weapon systems under discussion here could reach the European allies but not the United States does not in any way lesson our determination to do all that we can to limit and reduce what is -- in a broad sense -- a common threat."

Reagan reportedly expressed the "greatest admiration and esteem" for the courage and steadfastness he said Schmidt has shown in publicly supporting the NATO decision. The West German leader has threatened to resign if both parts of the decision are not carried through.

Reagan's letter is seen by Bonn government sources as the result of a discussion Schmidt had with the new U.S. ambassador to Bonn, Arthur Burns, shortly before Burns returned to Washington July 10. The West German leader reportedly told Burns that conflicting remarks by U.S. government officials about the NATO modernization plan were creating political difficulties here.