Shadow of Treason

During the Cold War, a RN warrant officer stationed in the British Embassy in Warsaw leaks secrets to his Polish girlfriend who's a Soviet agent and after his transfer to a naval station in Britain he joins a Soviet spy ring.
The film is based on the actual events of the Portland Spy Ring trial in the U.K. A disgruntled Navy Clerk is transferred to a secret research establishment and is subsequently black-mailed/paid by Czech intelligence to procure secrets for them. He seduces the secretary who controls the most secret documents, and they enjoy the fruits of their treachery until the British authorities begin to close in on them.