Mandatory voter identification (ID) requirements, which have historically been used to limit electoral participation, are becoming more common in developed democracies. In countries without a universal national ID, these laws create additional barriers to voting, potentially reducing electoral participation. Prior research on whether voter ID legislation affects turnout has yielded contradictory evidence, often lacks causal identification, and is predominantly centered on the US -- an exceptionally polarized context with a history of racialized voter suppression. To address these limitations, we investigate the recent introduction of mandatory voter ID in the UK, which does not share these exceptional features. Using evidence from this policy intervention, we conduct three analyses: (i) an evaluation of turnout changes at the local authority level before and after the reform, (ii) an analysis of individual-level self-reported turnout between voters with and without ID pre- and post-reform using panel data from the British Election Study (BES), and (iii) a causal estimation of the individual-level effect of ID ownership on turnout using an age-based discontinuity in photo driving license possession. Our analyses consistently show that the introduction of the voter ID law reduces electoral participation. These results highlight the suppressive effect of voter ID laws on turnout and the need for measures to mitigate this impact.