A secretary who has worked in the office of West German Chancellor Helmut Schmidt for the past 1 1/2 years, was detained by security authorities today on suspicion of spying for East Germany.

Although there was no immediate indication of how serious the case might be, its disclosure rattled one of the most dramatic political skeletons in West German politics.

It was three years ago last month that an East German spy - Guenther GUillaume - was uncovered among the top personal aides to Willy Brandt, then Chancellor. The revelation led to Brandt's resignation.

The case that began unfolding here today did not, on the surface, appear to be nearly so severe. It is certain, however, to create at least some additional embarrassment for Schmidt, who is already beleaguered by a host of political problems at home and who leaves Friday to attend a seven-nation summit meeting in London.

Manfred Schueler, who manages chancellery, affairs for Schmidt and oversees Bonn's counterintelligence agencies, told reporters that the suspected agent. Dagmar Kahlig-Scheffler, 30, was hired after she answered a newspaper advertisement for the job in December, 1975.

Normal security checks on her revealed nothing unusual, Schueler said.

The suspected spy, who is expected to be formally placed under arrest Friday, worked for almost all of her time in a department that deals mostly with Bonn's relations with the nine-member European Economic Community and with individual West European countries. She also reportedly dealt with documents on foreign aid.

Though the secretary did have access to material classified as "secret." Schueler said, her job did not put her in a position to jeopardize national security.

When the Guillaume affair exploded here in April 1974, it was quickly apparent that a very serious breach had been made in West German security. Guillaume admitted when arrested that he was an officer in the East German army.

He was a professional spy who came here along with tens of thousands of East German defectors in the mid-1950s and gradually penetrated to the seat of West German power, where he had access to numerous national security documents. He and his wife were sentenced to 13 and 8 years imprisonment respectively in December, 1975.

At the time of Guillaume's arrest, Brandt and hs Social Democratic Party were both suffering from sagging political prestige, not dissimilar from the troubles besetting Schmidt and his Social Democrats now.

Since the Guillaume affair, however, the West German security agencies have won higher marks from their allied colleagues for tightening up on security procedures and for making numerous key arrests of spy rings operating here.

West Germany traditionally has been a nest for agents from Eastern Europe, especially East Germans who blend in easily.