Federal Union is a Pro-European British group launched in November 1938, to advocate a Federal Union of Europe as a post-war aim. The founders of the organisation were Charles Kimber (1912-2008), Derek Rawnsley and Patrick Ransome. Other noted members of Federal Union included Harold Wilson,  Barbara Wootton, C. E. M. Joad, Stephen King-Hall and Philip Kerr, 11th Marquess of Lothian. In 1940 the group set up a Federal Union Research Institute (FURI), chaired by William Beveridge, to discuss the direction of post-war European integration. FURI attracted contributors from across the political spectrum, including  F.A. Hayek, J. B. Priestley, H. N. Brailsford, Lionel Robbins and Arnold Toynbee. In 1956 it argued for British participation in the European Economic Community. It continues to exist today, arguing for federalism for the whole of Europe and the world.

A federation (from Latin: foedus, gen.: foederis, "covenant"), also known as a federal state, is a political entity characterized by a union of partially self-governing states or regions under a central (federal) government. In a federation, the self-governing status of the component states, as well as the division of power between them and the central government, are typically constitutionally entrenched and may not be altered by a unilateral decision of either party, the states or the federal political body.