This article is the first sustained examination of Ben Nicholson's short-lived engagement with architectural painting. It analyses the three commissions he completed between 1949 and 1952 for large abstract panel paintings in new, distinctively modern forms of public or quasi-public interior space, which, it is suggested, collectively contributed to the post-war, social-democratic reform of ideas of Britain and Britishness. The article traces the development of Nicholson's thinking about the ideal relation of modern painting to modern architecture from the 1930s through to the execution of his panels, and further into the 1950s, when, during years of significant artistic and political change, he called for new architectural painting to be given greater independence from architecture. His changing attitudes are compared to the diverse views of his commissioning architects and shown to owe more to his understanding of early Italian wall painting and American abstract expressionism than to the British mural revival or the international architect-led campaign for a new "synthesis" of the arts. After illuminating his early awareness of large-scale American painting and of Clement Greenberg's concept of "a genre of painting located halfway between the easel and the mural", the article argues that Nicholson's panels exemplified a similar concept of a hybrid genre that side-stepped the heightened Cold War associations of easel painting with capitalism and mural painting with various forms of socialism, and presaged both the modernist retreat from the integrated mural and the ostensible depoliticisation of large-scale abstract painting.