Lasdun completed what may be regarded as his breakthrough masterwork in the Royal College of Physicians building in Regent's Park (1964). Inserted into a nationally important Nash set-piece terrace of neoclassical form, the RCP projects a raised linear form perpendicular to the terrace to create a series of gardens and spaces around the building, with annexes for lecture hall and an historic timber panelled room preserved from the earlier Colleges. Using modern reinforced concrete technology and highly expressive structural methods, the volumes are made to 'float in space' on the slimmest of supports to dissolve spatial boundaries between inside and out. The work makes implicit references to the work of the 'high modernists' from le Corbusier and Mies to the 'Scandinavian modernism' of Aalto, Asplund and Jacobsen as well as the contemporary Brutalist aesthetics of the era, yet developing a personal idiom of opened cantilevered volumes, long perspectives, triangulated form, and clarity of concept and structure that is entirely Lasdun's own. This building, however, is finished in luxurious white Sicilian marble, Murano glass mosaic tiles, polished brass and black engineering brick, and was one of the first post-War buildings to be awarded Grade I listing for national and international significance and influence on the work of others.