Moral legitimacy entails intrinsic value and helps executives convince firm’s stakeholders and the general public of the ethical acceptability of an institution or its activities or projects. Social license to operate is the social approval of those affected by a certain business activity, and it is receiving increasing attention, especially in the context of controversial projects such as mining and public works. Moral legitimacy provides ethical support to SLO. Drawing from the Aristotelian-Thomistic tradition and taking substantive justice and the common (...) good of society as the key references, this paper applies the Triple Font of Morality Theory and proposes four criteria which serve to evaluate moral legitimacy: contribution of the project or activity to the common good in a better way than other alternatives, morality of the means and procedures employed, ethical evaluation of the situation including stakeholder concerns and needs, and ethical evaluation of reasonably foreseeable consequences associated with the project and how to minimize possible damage or risks, and balance foreseeable negative consequences and benefits. The application of these criteria is illustrated through a project, presented as a case study, which certainly involved controversy and problems with SLO. The project was the construction of a rail tunnel for a high-speed train near the foundations of the Sagrada Familia, the well-known monumental church in Barcelona, Spain. (shrink)