Prime Minister Menachem Begin said today that the Israeli government had "no grounds to assume that atrocities against the civilian population would be perpetrated" when it authorized Lebanese Christian militia units to enter the Sabra and Shatila refugee camps of West Beirut.

In a letter to the judicial board of inquiry that is investigating the September massacre of hundreds of Palestinian refugees in the camps, Begin relied heavily on what he called "authoritative information" supplied to the Israeli Cabinet that some 2,000 heavily armed Palestinian guerrillas left in Beirut were "concentrated mainly in the camps of Sabra, Shatila and Fakhani."

"The picture drawn for us was one of difficult fighting, that the Lebanese Forces Christian Phalangist units would be compelled to battle the many terrorists dug in in those camps," Begin said in the letter. "Given this situation, it was never imagined that the Lebanese Forces, who are trained and organized military units, and who were assigned the task of fighting under difficult conditions, would want to--or be able to -- perpetrate a massacre."

Begin's letter contained no additional revelations, but his assertion that the Palestinian guerrillas were concentrated in the refugee camps raised a new question.

One of the areas that the inquiry board has not pursued in its public sessions is the fate of the 2,000 guerrillas said to have been in West Beirut at the time, and how Israeli military commanders expected a relatively small Phalangist force to defeat them in the camps. According to earlier testimony, between 100 and 130 Phalangist militiamen entered the camps on Sept. 16 -- meaning that if even only half of the 2,000 Palestinian guerrillas were in the refugee camps the Christian militiamen would have been outnumbered by almost 10 to one.

Along with making Begin's letter public today, the inquiry board also released letters from three other men. They are among the nine senior Israeli officials that the panel has warned may be harmed by the findings of the board, and have been given the choice to reappear before the board, submit new evidence or cross-examine other witnesses.

Begin declined the chance to reappear, saying his letter was his response to additional questions raised by the commission. Two of the others, Foreign Minister Yitzhak Shamir and Brig. Gen. Amos Yaron, said they would later submit written memorandums to the board. The other official whose letter was made public today, Maj. Gen. Amir Drori, asked to reappear before the inquiry board for new testimony.

The others who received warnings are Defense Minister Ariel Sharon, Army Chief of Staff Lt. Gen. Rafael Eitan, the director of military intelligence, Maj. Gen. Yehoshua Saguy; Sharon's personal assistant, Avi Dudai, and the head of Israel's intelligence service, Mossad, who by law cannot be named in public.

The commission has extended the deadline for replying to the warnings until Dec. 15.

In its warning to Begin, the inquiry board had told the prime minister he may be found guilty of "nonfulfillment of duty" because he "did not appropriately consider the role to be played by the Lebanese Forces during and due to the IDF's Israel Defense Forces' entry into West Beirut, and ignored the danger of acts of revenge and bloodshed by those forces against the population of the refugee camps."

Begin testified Nov. 8 that he had assumed "acts of revenge" would follow the assassination of Lebanese president-elect Bashir Gemayel but that he did not consider this a reason to prevent the Phalangist militia units from entering the Palestinian neighborhoods.

Begin reiterated that position today and sought to answer questions posed by commission members regarding warnings of bloodshed raised at a Cabinet meeting on Sept. 16 by Chief of Staff Eitan and Deputy Prime Minister David Levy.

Begin said Eitan's warning concerned the chances of widespread violence between Christians and Moslems throughout Lebanon following the Gemayel assassination and did not relate specifically to the refugee camps. He said neither Levy nor others at the Cabinet meeting suggested removing the Phalangist units from the camps, which in any case "did not appear feasible under the circumstances."

A spokesman for the inquiry board said the panel would schedule additional testimony as soon as possible after hearing from the other five officials by the Dec. 15 deadline.