Egyptian Vice President Hosni Mubarak has postponed a trip to Yougoslavia amid signs that President Tito has serious reservations about the proposed new peace treaty between Egypt and Israel.

Egyptian efforts to sell the treaty in Eastern Europe and the Third World centered on Yugoslavia and Romania -- two independent minded Communist countries that have played prominent roles on the sidelines of the Middle East conflict. Mubarak received a cordial welcome from Romania President Nicolas Ceausescu last week but failed to come to Yugoslavia to hand over details of the peace proposals to President Tito as earlier planned.

Both Egypt and the United States have kept Marshal Tito closely informed of the negotiations in the hope that he might help win support for the proposals within the nonaligned movement, of which he is a founding father.

One of President Carter's first acts on returning to Washington from the Middle East was to send a personal message to the 86-year-old Yugoslav leader.

So far, however, Tito has shown little desire to help persuade his friends in the Arab world to accept the treaty. Indeed, his attitude has been marked by extreme caution ever since President Anwar Sadat visited Jersualem in 1977.

Western diplomats here believe that President Tito's skepticism led to the cancelation of an earlier planned visit to Yugoslavia by Sadat. The official reason was that Tito was exhausted following extensive foreign travel, but the visit was never rescheduled.

Egyptian officials explaining the postponement of Mubarak's visit said he was unable to see Tito last Saturday as originally hoped because of the visit to Yugoslavia of Greek Prime Minister Constantine Karamanlis.

Whether or not this is true -- and Karamanlis' visit was announced several days before Mubarak's -- the fact remains that Tito has little reason to be seen shaking hands with the vice president of Egypt right now.

Over the last few months, the Yugoslav leader has been carefully cultivating his relations with key Arab states for a mixture of motives -- part economic, part political. He is hoping for Arab support to resist what is seen here as attempts by Cuba to turn the nonaligned movement, a key element in Yugoslavia's foreign policy, into an instrument of the Soviet Union.

Initial reaction from neighboring Romania, the only East European country to have diplomatic relations with Israel, has been more favorable. Romania welcomed the Camp David agreements as a possible first step on the road to a comprehensive peace.