Trade unions say they have collected enough signatures to force a nationwide vote aimed at preserving the network of state-owned post offices across the country.
Campaigners from the Communication union handed in more than 110,000 signatures for their initiative at the federal chancellery on Thursday.
The initiative, which is backed by the centre-left Social Democrats, seeks to halt any further opening up of the postal market to private competitors.
In the last few years Swiss Post has closed down about 900 offices mainly in rural regions. A further 400 offices are at risk.
The company said the initiative was counterproductive as it undermined plans for a more consumer-friendly service.
During its autumn session parliament is due to resume discussions on plans to fully open the mail market. The Senate has already come out in favour, while the House of Representatives is to tackle the issue later this month.
swissinfo.ch and agencies