Prime Minister Menachem Begin yesterday accused Egyptian President Anwar Sadat of breaking a promise made during his visit to Jerusalem that he would not issue war threats.

Speaking to reporters on his way to his office, Begin said, "Sadat said no more war when he was in Jerusalem and it wasn't qualified. This is very regrettable. He has broken his promise."

A Foreign Ministry spokesman said Sadat's second hard-line speech in two days, made to units of Egypt's 3rd Army in Suez, was viewed by top Israeli officials as not only a violation of the spirit of the historic Jerusalem peace talks, but as a break with the 1975 second Sinai disengagement pact in which both nations agreed not to wage or to threaten war.

Meanwhile, Defense Minister Ezer Weizman returned to Israel last night from a mysterious trip to London whose purpose was obscured by conflicting reports from Israeli officials.

The Foreign Ministry first said that Weizman went to London Wednesday for a "private affair." The Israeli Embassy in London later said that the defense minister had dinner with Sir Marcus Sieff, a leader of British Jewry and chairman of Marks and Spencer. He was also expected to meet another Jewish leader, at the same dinner, with Lord Rothschild officials said.

However, the offices of both mendenied to Israeli radio that Weizman dined with them, and Seiff's office said Sieff was out of the country at the time.

The Israeli Embassy in London finally conceded yesterday that Weizman could not have met with Seiff and Rothschild, adding only that the defense minister had flown from London to another European country - later reported to be Switzerland - to get a flight to Tel Aviv.

When Weizman arrived last night, he quickly got into a staff car and left the airport without talking to reporters.