A report to the British cabinet on the progress of research, begun in 1939, on utilizing the atomic energy of uranium for military purposes. The report is in two parts: Part I is a general statement about the experiments, methods, probable effect, materials and costs. Part II concerns the technical evidence supporting the statements of Part I, including the properties of uranium, the chain reaction properties of U235, size of the bomb, damage estimates, the size and characteristics of a separation plant to produce the bomb, and estimated costs. The report was transmitted to the United States government in 1941.