The Law and Justice (PiS)-sponsored amendment was backed by 233 MPs. 190 MPs of the 460-seat chamber voted against the bill.
All changes to the draft amendment proposed by opposition parties during parliamentary proceedings were rejected, all PiS-proposed changes were approved.
The Sejm passed, among others, PiS's proposal that higher posts in the civil service are to be filled by appointment rather in the course of contests.
In tune with the amendment work contracts of top-level civil servants will expire 30 days after the new law comes in force if they do not receive new job or wage conditions before that date.
The Sejm also backed a PiS-authored amendment to the Law on the Social Insurance System stating that the head of the ZUS national social insurer will be appointed by the prime minister at the request of the minister responsible for social security.
Earlier in the day, Civic Platform MPs said they plan to send the amended law to the Constitutional Tribunal (PAP)