1812/14 - 1977 British and Foreign State papers Main (Gardner) Stacks JX103 .A3 Bound 1(1812/14)-169(1967/68) and online at HeinOnline. Includes principal documents relating to the political and commercial affairs of nations and their relations with each other. It also contains texts of international treaties, constitutions, and other diplomatic documents such as statements, speeches, declarations, protocols and other communications relevant for international relations.
British Documents on Foreign Affairs -- Reports and Papers from the Foreign Office Confidential print
Part I, from the mid-nineteenth Century to the First World War
Part II, from the First to the Second World War
Part III, from 1940 through 1945
Part IV, from 1946 through 1950
Part V, from 1951 through 1956
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The following resources are selections of documents (mostly general correspondence) covering the build up to the First and Second World Wars and the Cold War.
British Documents on the Origins of the War, 1898-1914 Main (Gardner) Stacks D505 .G68 (11 volumes)
Documents on British Foreign Policy, 1919-1939 Main (Gardner) Stacks and NRLF (UCB) DA566.7 .A18 (62 volumes)
Documents on British Policy Overseas Main (Gardner) Stacks and NRLF (UCB) DA588 .D63 1984
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Additional resources
The National Archives (UK) provides two searchable indices to Foreign office correspondence:
Index to the Correspondence of the Foreign Office for the Year ... Main (Gardner) Stacks JX631.A12 A3