Prior studies have examined the impact of retirement on health, but there is no consensus as to whether longer working lives lead to better health. We investigate the impact of a recent pension reform which, since 2010, gradually extended the age of eligibility for the State Pension for women in the UK, from 60 to 66 years. Using a difference-in-differences approach on 8400 observations of women aged 60-64 participating in the Understanding Society study from 2009 to 2015, we find strong evidence that increasing the State Pension Age led to a statistically and clinically significant negative effect on mental and physical health. We also find evidence that women with a longer extension in the State Pension Age experienced worse health outcomes. We show that this effect is driven by women from routine-manual occupations and led to increasing health inequalities between occupational groups.