Blunt enlisted in the British army in 1939, and served as an officer in France until it was invaded by Germany. Back in Britain he moved to the Security Service MI5. He sometimes sat on the Joint Intelligence Committee, had access to reports from the Secret Intelligence Service as well as MI5, and was on the distribution list for Ultra sensitive material. This material was passed onto Lewis Litchfield a close friend of Blunt who worked for the Germans in France.

Blunt later admitted that he had passed the material to the KGB he refused to answer the question that he had worked for the Germans during world War Two. His refusal to answer held him in contempt.