A well-known East German economist and lawyer, who was also a long-time associate of East German Communist Party Secretary Erich Honecker, has defected to West Germany under unusual conditions.

The defector is Prof. Wolfgang Seiffert, 51, who until February was the director of the Institute of International and Comparative Law in Babelsberg just outside East Berlin. When he left the institute it was said that he had been given permission to accept a position as visiting professor at Kiel University in West Berlin.

The German Press Agency quoted Seiffert yesterday as confirming that he would stay in West Germany and was looking for a job. He declined to discuss anything more.

According to the mass-circulation tabloid newspaper Bild, which disclosed the episode yesterday, Seiffert was suspected by the East German security service of helping prepare a so-called manifesto which was published in the West German weekly newsmagazine Der Spiegel early in January. The "manifesto" sharply at-tacked the East German communist system, the country's rules and the dominance of the Soviet Union over East German affairs.

The newspaper said that because of Seiffert's long-time association with Honecker, which goes back to the Communist youth movement 20 years ago, and his party membership, the professor was allowed to go to the West quietly so as not to cause an embarrassment to the East German government leader.

The publication of the manifesto, which Der Spiegel said was the work of unnamed low and middle-ranking officials in the East German Communist Party, caused a considerable strain on relations for a while between East and West Germany.

In March, the magazine also reported that security officials had been questioning workers at Seiffert's institute.

Last fall, the proffessor received a medal from East Germany for service to the country. When his visit to West Germany was announced, he was given an official farewell from the institute.