Czerniawski graduated in the late 1930s from the Wyższa Szkoła Wojenna (WSWoj), a military academy at Warsaw. As a former officer of the Polish Air Force, he volunteered to create an allied espionage network in France in 1940. He set it up with Mathilde Carré who recruited the agents; some French declined to work for a Pole. This network was code-named Interallie.
Czerniawski was evacuated to Britain to be examined by Polish intelligence and then meet General Sikorski where he was presented with the Virtuti Militari. He was returned to France by parachute in November 1941.
On November 17, 1941, the Abwehr group of Hugo Bleicher arrested Czerniawski and then Carré. The network had been uncovered due to the lack of proper operational security within the organization, and many other members of the Interallie were picked up after Carré agreed to cooperate with the Germans in return for her life. Czerniawski and others were imprisoned.
After having been offered safety by the Germans, he was sent to England as an agent. However, he made himself known to the British authorities. He was de-briefed by the British (MI6) and Polish authorities about the security lapses of his organization in France. He was then employed as a double agent by MI5 using the codename "Brutus" (after Caesar's friend and assassin) under their Double Cross System. His strong anti-Russian attitude, manifested in his denouncing (in a pamphlet he authored) a Polish officer who attended an official reception at the Soviet Embassy, led to doubts about his suitability. For this act of mutiny against the Polish authorities, he was arrested and imprisoned. MI5 produced a cover story that he had been detained in a sweep of "anti-Bolshevik" Poles.
A Polish court martial found him guilty of gross insubordination, but to keep the matter quiet sentenced him to only two months imprisonment. After his release from prison, Czerniawski was unrepentant to his handlers; MI5 doubted his reliability, thinking him fickle and liable to meddle, and MI5 also harbored concerns that the Germans would be suspicious about his arrest and swift release. He was no longer permitted to operate the radio himself and he was only used for distribution of low grade information ("chicken feed"). Initial German suspicion faded and in December 1943 the British decided to use Brutus for distribution of important deception information.
As such, he played a major part in the allied deception prior to the D-Day landings in Normandy in 1944 as one of the primary agents passing false information as part of Fortitude South, the deception plan aimed at convincing Germany that the Allies would invade Europe in the Pas de Calais area across the English Channel from South-East England.
After the war he stayed in the UK and wrote The Big Network, published in 1961.
- Andrzej Pepłoński, Wywiad Polskich Sił Zbrojnych na Zachodzie, 1939-1945 (Polish Armed Forces Intelligence in the West, 1939–1945), Warsaw, 1995.
- Stanisław Żochowski, Wywiad polski we Francji 1940-1945 (Polish Intelligence in France, 1940–1945), Lublin, 1994, ISBN 83-902348-5-8.
- John Cecil Masterman, The Double-Cross System in the War of 1939-1945, Yale University Press, 1972.
- Macintyre, Ben (2013). Double Cross: The True Story of the D-Day Spies. Broadway Books. ISBN 978-0307888778.
- Garby-Czerniawski, Roman (1961). The Big Network. London: George Ronald.
- Macintyre p46-48
- McIntyre p136-137
- MacIntyre p191-192
- McIntyre p195-196
- "Niebieska eskadra - groby, cmentarze, pomniki, miejsca pamięci polskich lotników - Roman Czerniawski" (in Polish). Retrieved 2014-09-04.