He then served as Legationsrat (counsellor) in the London Embassy under Ambassador Joachim von Ribbentrop for whom he developed a personal dislike and a professional disdain.
Both Erich Kordt and his brother, Theodor, played a part in the Oster Conspiracy of 1938, which was a proposed plan to assassinate Adolf Hitler if Germany went to war with Czechoslovakia over the Sudetenland.
However, in the event, British Prime Minister Neville Chamberlain was apprehensive of the possibility of war and so negotiated interminably with Hitler and eventually conceded to him.
In June 1939, Kordt went to London to warn Robert Vansittart, the diplomatic advisor to the British government, of the secret negotiations between Germany and the Soviet Union which were to lead to the Nazi-Soviet Pact.
In June 1946, at the Nuremberg Trials, Kordt testified on behalf of Ernst von Weizsäcker, State Secretary of the Foreign Ministry of Nazi Germany, and later German ambassador to the Vatican.