Newspapers in Swiss media group Tamedia first reported the initiative Service Citoyen (Citizen Service) has filed a criminal complaint alleging malfeasance that could have skewed results in Swiss elections.
The disclosure has jolted Switzerland's vaunted system of direct democracy, where postal voting is the main way that people cast their ballots and where referendums four times a year give citizens a direct say in policymaking.
The next such referendum is on September 22, when a biodiversity project is among issues on the ballot.
Advocacy group co-president Noemie Roten commissioned an outside firm last year to collect some 10,000 signatures on electoral registers, and she subsequently discovered that nearly a third appeared fake, involving bogus addresses, falsified birth dates and repeated signatures.
She issued a 236-page report to the Swiss attorney-general's office, which said it was investigating the matter.
Roten said the findings "could indeed jeopardise the confidence that Swiss citizens have in direct democracy".
The Swiss federal chancellery, which oversees national elections, did not immediately respond to a request for comment from the AP about the possible impact of the suspected wrongdoing.
