Egyptian President Hosni Mubarak, using language that appeared to dim further the waning hopes for progress toward Mideast peace, told President Reagan yesterday that the Palestinian people must have the right to self-determination and "a national entity" of their own.

"The key to peace and stability in the area is to solve the Palestinian problem," Mubarak said as he arrived at the White House for his first visit since becoming president of Egypt after the assassination of Anwar Sadat last October.

His choice of words underscored anew that there appears to be little chance for a quick breakthrough in the Egyptian-Israeli negotiations over autonomy for the 1.3 million Palestinian inhabitants of Israeli-occupied territories.

That has been clear to U.S. policy makers since Secretary of State Alexander M. Haig Jr. returned last week from a Middle East mission aimed at gauging the prospects for an autonomy agreement timed to Israel's scheduled return of the Sinai Peninsula to Egypt at the end of April. Yesterday, Mubarak seemed intent on making that point publicly in Reagan's presence.

Mubarak, who says he wants to heal the breach between Egypt and the rest of the Arab world, appears determined not to be put in a position where he could be charged with selling out Palestinian interests.

While his call for "a national entity" seemed to stop short of advocating an independent Palestinian state, it also went far beyond anything Israel is willing to permit in the West Bank and Gaza Strip territories.

But, although he made clear that the gulf between the Israeli and Egyptian positions remains very wide, Mubarak reiterated the promise he had given to Haig to cooperate in efforts to accelerate the autonomy talks that were launched 20 months ago under the Camp David accords.

After the meeting between Mubarak and Reagan a senior administration official emphasized their agreement on that point, and said they also had agreed that the Camp David process is "the only appropriate vehicle" for pursuing an autonomy accord.

Earlier, Reagan, in welcoming Mubarak, also sought to stress that the U.S.-mediated peace process launched at Camp David has not reached the end of its effectiveness, as many critics in the Arab world and Western Europe say. The president said:

"We must commit ourselves to push on . . . because no matter how long and arduous it offers the best opportunity for tangible results. Without setting deadlines, I personally believe the time has come to get on with the task before us, and the sooner the better."

The senior official, who declined to be identified, refused to speculate on what Mubarak meant by the term "national entity." Later, when Mubarak was asked for a definition after a luncheon with Haig at the State Department, he replied that he had "stated all my conception concerning the Palestinian state" in his remarks at the White House and would not comment further.

Although Reagan and Mubarak went out of their way to stress their desire for continued close relations, Mubarak's visit is regarded as potentially pivotal in determining whether Egypt remains on the course of close alignment with Washington, charted by Sadat, or whether Mubarak moves toward greater independence in his foreign relations.

Egypt is second only to Israel as the largest recipient of U.S. aid, and the administration plans to seek an increase of $400 million in military assistance to Mubarak in the next fiscal year.

Nevertheless, Mubarak is known to feel that the United States should do more in granting loans that do not have to be repaid, in accelerating deliveries of military equipment and in putting aid to Egypt on a par with that granted to Israel.

Reagan, in his remarks, put great stress on the threat posed to the Middle East by the Soviet Union and the need for continued close U.S.-Egyptian cooperation on regional security.

The senior official later said the two leaders had "reconfirmed our identity of views" on Mideast security issues. Mubarak, though, said only that the subject had been discussed.