Detailed files were kept on thousands of East Germans.
The figures, released by the office responsible for the German state of Thuringia's Stasi files, are based on a study by historian Helmut Müller-Enbergs to be published Thursday. Until now, historians had estimated the Stasi had 174,000 informants.
According to the study, about one in 20 members of the former East German Communist party, the SED, was a secret police informant. Over the regime's 51 year history the number of civilian spies added up to about 620,000, including 12,000 West Germans who worked for the Communist dictatorship. The majority of informants were men, between the ages of 25 and 40.
For most Stasi informers the prime motivation, the study found, was a belief in the Communist state's political ideology. Financial reward played only a minor role and blackmail was rare, it said.
The Stasi infiltrated nearly every aspect of life in the GDR. Its network of professional and civilian spies monitored and collected information on large swathes of the population.