Prime Minister Menachem Begin, citing unspecified "personal reasons," today canceled his scheduled trip to Washington next week to see President Reagan.

According to a brief statement issued late this afternoon by his office, Begin telephoned Reagan this morning and told him "that for personal reasons he will have to postpone his visit.

"The president expressed his understanding," the statement said. "He agreed that they would think in terms of rescheduling the visit to some time later this year."

In Washington, the White House issued a statement to the same effect and indicated that the administration anticipates that "discussions on the Middle East peace process with Israel and other nations in the region will continue."

Aides to Begin refused to elaborate on the "personal reasons," but the prime minister's spokesman, Uri Porat, insisted that they had nothing to do with Begin's physical or emotional health, a subject of much speculation here.

Another senior Israeli official said Begin was never enthusiastic about making the visit now and saw no particular purpose in going through with it at a time when U.S.-Israeli relations are on an upswing anyway. This coupled with Begin's admittedly melancholy mood since the death in November of his wife, Aliza, may have led to the cancellation, the official speculated.

U.S. officials here clearly were caught off guard by the decision. Only hours before the announcement, a U.S. official said planning for the Begin-Reagan meeting at the White House next Wednesday was going forward and outlined some of the likely topics of discussion between the two men.

He said these included solidifying the long-term U.S.-Israeli relationship, the futile efforts so far to gain a Syrian troop withdrawal from Lebanon and Israel's plans for a partial withdrawal of its troops to new positions in southern Lebanon.

Lebanese President Amin Gemayel is scheduled to meet with Reagan at the White House Friday to discuss some of the same issues.

Despite Porat's public insistence that there was no connection between the cancellation and Begin's health, speculation along those lines appeared certain to increase. Shimon Peres, the leader of the opposition Labor Alignment, said in a statement tonight that the decision to cancel the visit should cause Israelis concern "about the functioning of the prime minister and worry that the government is not in firm hands."

Begin, who will celebrate his 70th birthday this weekend, is in frail physical health. During the past several years he has suffered two heart attacks and a mild stroke. Moreover, he has been in a subdued, some have said "deeply depressed" mood, since the death of his wife.

In the months since then Begin has made few public appearances or speeches, sometimes canceling participation in a scheduled event at the last moment as he did recently in refusing to appear before officials of the Jewish Agency.

According to officials who see him frequently, Begin no longer dominates private meetings and now listens to others more than he speaks. They use terms such as "subdued" to described his manner but say there is no evidence that Begin is any less alert than in the past. He was described as "very alert" during Secretary of State George P. Shultz's stop in Jerusalem earlier this month.

"I know it is not health," a senior official with access to Begin said of today's announcement. "He's healthy enough to make the trip. He's not in a deep depression. He's not cheerful, that's true, but he's not in a depressed mood."

The official said, however, that Begin's frail physical health and emotional mood may have contributed to his lack of enthusiasm for making the trip now. He said Begin "had doubts all along" about the timing of his visit to Washington and felt that there was not that much that urgently needed to be discussed by him and Reagan.

Israeli officials said they were confident that the Begin-Reagan meeting would be rescheduled later this year, probably in October or November, and predicted that the cancellation would have no impact on relations between the two countries or on Israeli plans to "redeploy" troops in Lebanon.

"This will not affect the relationship," one official said. "I think there is now a deepening understanding in the United States about the unreliability of the Arabs. We can feel the administration coming back to the view of Israel as a strategic asset and ally in the Middle East."

Today's announcement came as a surprise, but the decision apparently was not abrupt. Even before the announcement, there had been some speculation here that a cancellation might be in the offing because of the unusual way in which planning for the trip was proceeding.

As late as yesterday, spokesmen for both Defense Minister Moshe Arens and Foreign Minister Yitzhak Shamir said the two senior ministers had not yet been informed whether they were to accompany Begin on the journey. Begin's office also withheld all details of his travel plans, although normally they would have been public by now.

According to an informed official, last week when Begin's aides suggested dispatching an advance team to Washington--a normal procedure before such a trip--the prime minister told them not to bother, that the Israeli Embassy in Washington could handle all the details.

A senior aide to Begin conceded that the possibility of a cancellation had been known for several days but that the decision was not finally made until a meeting in Begin's office today.

One official suggested that with things going well in U.S.-Israeli relations from the Israeli point of view, Begin may have seen no point in a face-to-face meeting with the president at which issues that still divide the countries might be raised. Those issues include Reagan's call last fall for a freeze on Jewish settlements in the occupied West Bank that Begin rejected out of hand at the time.

Begin and the president last met in June 1982, shortly after Israel's invasion of Lebanon. They were to meet again in November but Aliza Begin's death intervened. The meeting was rescheduled for February, but by then the administration was angry with Israel at the lack of progress in the Lebanon negotiations, and the president made it known that Begin should not come until Israel agreed to withdraw its troops from Lebanon.