This movie was released thirteen years after its source novel, "The Strange Case of Mr. Pelham" by Anthony Armstrong, had been published, and fifteen years after that novel's source story, "The Case of Mr. Pelham", written also by Armstrong, had been used for an episode of Alfred Hitchcock Presents (1955).
According to the website SyFy.com, "Basil Dearden, the director, died shortly after completing filming, dying in a car crash in a place that was in the exact same location that a major character dies in the film."
Sir Roger Moore is most famous for playing James Bond (in seven movies between 1973 and 1985). This movie was released three years before his first Bond movie, Live and Let Die (1973), but coincidentally his character says in one scene "Espionage isn't all James Bond on Her Majesty's Secret Service."
Two stills from the movie, one showing Harold Pelham (Sir Roger Moore) reaching across for his seat belt, and the other showing a close-up of his hand fastening the seat belt buckle, were used as part of a road safety campaign to persuade people always to wear their seat belts.