In this paper we analyse whether entitlement to a free part-time place in a nursery or other registered setting at 3 years of age affects educational attainment in primary school. Our identification strategy exploits date-of-birth discontinuities in eligibility for free pre-school. In England, children are entitled to a free part-time pre-school place the term after they turn 3, while they usually start school in the month of September of the academic year in which they turn 5. This implies that some children born just a few days apart are entitled to different amounts of pre-school education (up to 3 months) while starting school at the same time and within the same cohort. Using a regression discontinuity approach and analysing administrative data on all state school pupils in England, we estimate that eligibility to free part-time pre-school education results in a small overall educational advantage for boys at age 5, but not for girls. Analysis by subgroups reveals that the effects of eligibility are unequally distributed across children from low and high income families, and contribute to increase the socio-economic gap for both boys and girls at age 5 and (to a lesser extent) at age 7. As survey data from a recent cohort of English children (Millennium Cohort Study) indicates that most children receive formal childcare well before their entitlement date, and that the eligibility rules have a small impact on actual enrolment, our estimates could be interpreted as a lower bound of the effect of pre-school education on early attainment.