Public Health Wales has apologised after confirming almost 400 smear tests were contaminated by out of date sample pots.
Cervical Screening Wales has written to the 369 affected women to alert them to the issue.
The programme, run by Public Health Wales, said a small percentage of smear test samples were sent for screening in sample pots that were out of date.
Smear tests are carried out at GP practices, sexual health clinics and hospital clinics. The cervical sample is put into a vial containing preservative fluid, and returned to a laboratory for screening.
But in 369 cases the preservative fluid used in the vials had expired.
Some of the affected samples have already been screened again. Cervical Screening Wales are reviewing these test results and contacting the women affected to offer advice.
It also confirmed women who were screened for HPV, a virus which acuses almost all cases of cervical cancer, must be retested as their existing samples cannot be rescreened.
The issue affects fewer than two per cent of women who received cervical screening in Wales between July and October 2018.
Public Health Wales have apologised for the error.