Israel's opposition Labor Party has mounted an uncommonly scathing verbal attack on Prime Minister Menachem Begin, charging that his refusal to permit contacts between labor leaders Shimon Peres and Arab leaders reflects a "sickness" possibly resulting from over-medication.

The Labor alignment plans to introduce a motion of no confidence in parliament next week, a prospect that Begin brushed aside nonchalantly by saying, "It will be very interesting, really."

The criticism followed by an angry outburst by Begin over Peres' contacts with Arab leaders and reports that Peres planned more such contacts.

Former prime minister Golda Meir, responding to Begin's handling of the controversies, said that in the past week Begin - who has been under treatment for heart trouble - has not appeared to be in full control of his senses.

"The sickness cannot justify anything," Meir said. "When a close friend comes to your house and ruins it all, you can't just stand and watch it with equanimity."

Former finance minister Yehoshua Rabinowitz was quoted as saying at a Labor Party meeting Thursday that Begin's Knesset appearace Wednesday "was that a person who is not normal. The joy, the gaiety - it frightens me." In the Knesset Begin exploded with anger over Peres' meeting with Egyptian President Anwar Sadat and ripped a paper in half.

Shlomo Hillel, former police minister, told the Labor group, "I am fully convinced that a good part of [the] phenomena was a result of medications."

The Labor Party issued a statement expressing "deep shock" over what it termed Begin's "irresponsible and unchecked expressions."

Following the Labor Party attack, leaders of Begin's Likud coalition held an emergency meeting and issued a statement calling the accusations "slanderous."

At a meeting of the National Religious Party Thursday night, Begin said: "Someone is trying to compete with us and conduct the peace negotiations."

Many of the remarks by the Labor Party figures were similar in tone to those made by hecklers on the Knesset floor who attempted to shout Begin down when he argued with Peres over the Sadat meeting in Vienna on July 9.

In the past, opposition politicians have suggested publicly that Begin may not be physically capable of withstanding the pressures of the intensive Middle East peace negotiation process. Previously, however, they had not coordinated their attacks as they did following the emotional Knesset debate.

Since suffering a series of heart ailments that began just before his election last year, Begin has been the object of media speculation that his behavior is affected by contrasting medications he takes for his heart condition and for treatment of diabetes. Begin's physicians have denied these suggestions, saying the prime minister is in good health.

Meanwhile, Foreign Minister Moshe Dayan said last night that Israel is willing to modify its 26-point peace plan in negotiations with Egypt, but not before the two nations resume peace talks.

In a tone that appeared more conciliatory than recent Israeli government pronouncements, Dayan said: "Our plan is not the Bible. It is not the Gospel." His remarks came in an interview on Israel television.

He suggested there is room for agreement in the question of Palestinian refugees and a possibility of Israel being more flexible in its insistence on maintaining a military presence in the occuped West Bank and Gaza Strip.

"I sincerely believe there is a need to solve the question of refugees, because there will not be an end to terrorism unless we solve the refugee question," Dayan said. But he said a necessary first step would be to identify what Middle East countries would be willing to accept refugees, and then negotiate their relocation.

Dayan said that because the government "wants to see the end of the road," he favors a comprehensive settlement as opposed to an interim solution. He added that Israel will continue to insist on two points: No Arab sovereignty in the occupied West Bank and no prohibition of Jewish settlements there.

But Dayan quickly added, "We do not insist on Israeli sovereignty." Under the Israeli plan, Palestinian Arabs would gain limited self-rule, while Israel would retain Jewish settlements and some military presence.

While the government has maintained since peace negotiations broke off in January that it was willing to negotiate any aspect of either Israel's peace plan of Egypt's, Dayan's remarks contrasted sharply with official Israeli statements leading up to this week's foreign ministers' conference at Leeds Castle, England.

Before the conference, Israel rejected the peace plan put forward by Egyptian President Anwar Sadat, and Israeli officials repeatedly cited "wide gaps" between the two plans, which they doubted could be overcome without major changes in the Egyptian position.