Your reporter David Leitch says that a new book, The Fifth Man by Roland Perry, claims that Lord (Victor) Rothschild 'first alerted Stalin to Allied plans to build an atom bomb using Plutonium 235'. Perry alleges that Rothschild passed on to Russian intelligence early details (1943) on the plutonium bomb. Mr Leitch seems to have confused this with the bomb which used Uranium atom 235 as its fuel. This was more difficult to collect, and came later.
Perry mounts parts of his case on the claim that Rothschild was first to pass on data on the plutonium bomb being developed at Professor G P Thompson's laboratory at Imperial College. Certainly, this would have put him ahead of that other KGB spy, Klaus Fuchs, who was labouring with limited success on the U235 route at Sir Mark Oliphant's laboratories at Birmingham University. Fuchs had little success until he joined the Manhattan Project at Los Alamos in the United States, late in 1944.
Mr Leitch further writes that the KGB were grateful to John Cairncross, the Treasury spy, for 'atom secrets'. In his book KGB, Christopher Andrew gives 'probable' credit to Cairncross, in the summer of 1941, for telling the Russians that there was a joint Anglo-American decision to build an atomic bomb. This hardly amounted to 'atom secrets'. At that time, there were quite simply no great technical secrets to tell. The efforts to create such a weapon had not yet begun.
Richard Joslin London SW18