Ernst Constantin Fetterlein (3 April 1873[1] – June 1944[2][3]) was a Russian cryptographer who later defected to Britain.
[5] During World War I, he was known for a time as Ernst Popov as his German-derived name could have drawn unwanted attention.
[5] Upon the Russian Revolution of 1917, he fled to Western Europe with his wife on board a Swedish ship, narrowly evading capture.
After the end of World War I, he worked for the successor to Room 40, the Government Code and Cypher School (GC&CS), becoming a senior assistant on 17 December 1919.
[5] Fetterlein came out of retirement during World War II to assist GC&CS's diplomatic section at Berkeley Street.