Research shows that children’s block construction skills are positively associated with their concurrent and later mathematics performance (e.g., Nath & Szucs, 2014; Verdine et al., 2014). Furthermore, there is evidence that block construction training is particularly beneficial for improving early mathematics skills in children from low-Socio Economic Status (SES) groups who are known to have lower maths performance than their peers (Bower et al. 2020a; Dickerson & Popli, 2016). This study investigates the association between block construction and mathematics in children who are approaching the age of formal education in the UK (4 years). It also investigates whether the relation between block construction and mathematics differs between children from more compared to less affluent families. Participants included 116 children (M = 3 years 11 months, SD = 3 months). All participants completed numeracy, block construction, and receptive vocabulary tasks. Socio-economic status and demographic information (child age, gender, ethnicity) were also obtained from parents. Regression analysis showed block construction accuracy explained 5.3% of the variation in numeracy after controlling for covariates. When separated by SES group, for children from less affluent families, block construction explained 14.9% of the variation in numeracy after covariates. For children from more affluent families, block construction explained no additional variation in numeracy. These findings highlight one possible intervention target for promoting maths-readiness in children of this age. More specifically, interventions involving block construction skills may help to reduce attainment gaps in UK children’s mathematics achievement.