Secretary of State Alexander M. Haig Jr. has concluded that there is no realistic possibility of an Israeli-Egyptian agreement on Palestinian autonomy before Israel returns the final occupied portion of the Sinai Peninsula to Egypt on April 25.

Haig's assessment was made known as he flew home from his second Middle East mission in two weeks. These talks, centering on the stalled autonomy negotiations, amounted to the first intense involvement by Haig and the Reagan administration in the difficult diplomacy of the Arab-Israeli dispute.

Haig arrived in Washington Friday night.

Far from being discouraged by what he found, Haig appears to be satisfied that his introduction to Middle East diplomacy was successful in halting and reversing movement toward mistrust and antipathy between Israel and Egypt. Reporters aboard his plane were told this rising tension, in Haig's view, had not only blocked progress in the autonomy negotiations but also endangered the political basis for the hard-won peace between the two former enemies.

Part of the reason for this buildup of tension, officials conceded, was the relative inactivity of the United States in Middle East diplomacy during recent months. Haig had planned to become involved last fall but he was impeded by other difficulties, including congressional resistance to the sale of reconnaissance planes to Saudi Arabia, the assassination of Egyptian president Anwar Sadat and Israel's annexation of the Golan Heights.

Uncertainty in Israel about Egypt's policies after the return of the Sinai and in Egypt about whether Israel would give back the land was compounded by uncertainty in both countries about the Reagan administration's intentions and role, officials say.

Haig believes his trip two weeks ago to "find facts" in the autonomy dispute and his just-completed trip to convey a U.S. assessment and preliminary recommendations helped to eliminate this uncertainty, according to the senior official who briefed reporters.

Haig brings a few tangible achievements back with him. During discussions with Haig in Jerusalem, Israeli Prime Minister Menachem Begin is said to have finally approved the basis for the participation by Britain, France, the Netherlands and Italy in the multinational force to police the demilitarized Sinai after Israel returns it to Egypt.

Participation of the European nations had been held up for many weeks by a dispute with Israel about the wording of their agreements to participate. The Israeli Cabinet is now expected to give formal approval Sunday to European participation.

A senior Israeli source told Washington Post correspondent William Claiborne in Jerusalem that Haig had given Begin "clarifications" on the positions of the four European countries. The clarifications, he said, appeared to meet most of Israel's requests that participation in the multinational force be based solely on the Camp David accords. The source said that a Cabinet decision could be postponed another week if any ministers raise further objections to the European position.

The source also said that when Haig returns to Washington he is expected to convey to Begin the results of his meetings in Cairo and that the Israeli Cabinet would then determine "how to proceed from a technical point of view" on the Palestinian autonomy negotiations.

The Israeli official added, "We suggested intensive negotiations from now on and we still hope that we can get an agreement before April 25. We will have to wait to see what reports we get from the United States and proceed from that point."

Haig was also said to be about to resolve a dispute over navigational rights in the Strait of Tiran, which connects the Gulf of Aqaba with the Red Sea. This will be guaranteed by the multinational force, according to the resolution worked out in recent days.

As part of the assurance to Egypt, the Reagan administration is preparing to increase substantially U.S. military aid to that country. After being around $500 million annually for several years, military aid to Egypt was raised to $900 million in the current budget.

Now, in response to pleas from Egyptian President Hosni Mubarak, a further increase to about $1.3 billion is likely. Mubarak, who is to visit to Washington next week, is nevertheless not completely happy with the terms of the aid.

As for the Palestinian autonomy negotiations, Haig is claiming only slight progress. "It's been slow," he said on leaving Cairo this morning, adding, "We have far more to do than we have done already."

More significant, in Haig's view, is his belief that the focus of the autonomy discussions has shifted from the examination of differences to a search for solutions.

Haig is also expressing hope, but not confidence, that the current activity, especially in the focus of attention before the Sinai return, can succeed in removing or sharply reducing some of the longstanding differences. But he has no doubt now that Palestinian autonomy is a tremendously difficult problem, reporters on his plane were told.

To this end, Haig has picked Richard Fairbanks, who served as chief of congressional relations for the State Department during Haig's first year, as special U.S. negotiator for autonomy.

Haig is holding out the possibility of further direct involvement and future trips to move the negotiations along, if they are needed, but he has no trips immediately in prospect.