She was, her wartime colleagues said, “one of the finest members of the service.” For over six years Krystyna Skarbek had risked her life for Britain’s Special Operations Executive, saving the lives of several fellow officers and stunning the head of SOE with her “spirit and courage”.

This Polish-born countess, under her nom-de-guerre of Christine Granville, was not only the first but also the longest-serving female special agent of the Second World War. She won an OBE, a George Medal, the French Croix de Guerre.

And until very recently, almost nobody knew what she did.

Christine was en route to her second husband’s diplomatic posting in southern Africa when the Nazis invaded her homeland in September 1939. She was so incensed she demanded the British secret services take her on immediately.