Background Research on inequalities in cervical cancer screening (CCS) participation has overlooked the distinction between ‘never-' and ‘under-screeners' while different socioeconomic and demographic determinants may underlie ‘non-' and ‘under-' screening participation. This study examines socioeconomic and demographic inequalities in never and under CCS participation. We compare cross-national prevalence and trends among these two groups in Switzerland and Belgium, two countries with similar opportunistic CCS strategy but different healthcare systems. Methods Data on 38,806 women aged 20–70 from the Swiss Health Interview Survey (1992–2012) and 19,019 women aged 25–64 from the Belgian Health Interview Survey (1997–2013), both population-based cross-sectional nationally representative surveys, was analysed. Weighted adjusted prevalence ratios were estimated with multivariate Poisson regressions. Results Over the studied period, never screening prevalence was about 15% in both Switzerland and Belgium and under screening prevalence about 14.0%. Socioeconomic gradients were found among both never- and under-screeners. Higher income women had lower never and under screening prevalence in Switzerland and a similar gradient in education was observed in Belgium. Importantly, distinct socioeconomic and demographic determinants were found to underlie never and under screening participation. Never screening was significantly higher among foreign nationals in both countries and this association was not observed in under screening. Never screening prevalence was lower among older age groups, while under screening increased with older age. Over time, age inequalities diminished among never- and under- screeners in Switzerland while educational inequalities increased among never-screeners in Belgium. Conclusion Findings revealed that determinants of screening inequalities differed among never- and under-screeners and hence these should be addressed with different public health strategies. Crucially, socioeconomic and demographic inequalities were more pronounced among never-screeners who appeared to face more structural and persistent inequalities. Differences between the two countries should also be noted. The more liberal-type Swiss healthcare systems appeared to shape income-related screening inequalities, while education appeared to be a stronger determinant of never- and under-screening in Belgium.