© Author(s) (or their employer(s)) 2018. No commercial re-use. See rights and permissions. Published by BMJ. Background The principle of equivalence in prison health has been established for nearly four decades. It seeks to ensure that prisoners have access to the same level of healthcare as members of society at large, which is entrenched within the international legal framework and England's national health policies. Aims This study examined how key policymakers interpret and implement the principle of equivalence in English prisons. It also identified opportunities and threats associated with the application of the principle. Methods In total, 30 policymakers took part in this research. These participants engaged in policymaking activities and occupied positions of authority in the prison field. Results Despite the policymakers' consensus on the importance of the equivalence principle, there was a varying degree of understanding regarding what constitutes 'equivalence'. Participants described how the security culture impedes prisoners' access to healthcare services. Additionally, the increasing size and complexity of the prison population, coupled with a diminishing level of resources, reduce the level of care being provided in prisons and thus compromise implementation of equivalence in English prisons. Conclusions Inconsistent interpretation of equivalence, the prevailing security drive, increasing numbers and health complexities of prisoners and fiscal austerity threaten the implementation of equivalence in English prisons. This research calls for new guidance on how to interpret and implement equivalence, along with measures to educate prison governors and staff on the prison rehabilitation value, ensure greater investment in prison health and consider alternatives to imprisonment to future-proof the principle of equivalence in the English prison system.