It seems late in the day still to be discovering radically different concentration camp stories. Yet this biography of a Jewish mathematician who masqueraded as a Polish noblewoman and infiltrated an evil system in order to undermine it is a truly extraordinary story, and all the more so for having nearly been lost to history. If Nicky Winton (the British stockbroker and subject of the recent film One Life) is rightly lauded for having saved the lives of 669 (mainly) Czech children via his Kindertransport scheme, Janina Mehlberg is estimated to have saved more than 10,000 mainly Polish camp inmates. While One Life upholds the Talmudic Golden Rule that ‘to save one life is to save the world entire’, Janina took as her guiding principle ‘to save one life is always less than saving many’.

A professional mathematician born in 1915 in the Galician town of Zurawwno, Janina took numbers as her primary value, counting her own life insignificant compared to those of the thousands she would rescue.

Subterfuge: Janina with her husband Henryk Mehlberg

To this end, she reinvented herself. At the age of 18 she married fellow student Henryk Mehlberg. At the Nazi invasion, family friend Count Skvzynski ennobled her with the title of Countess Sucholska, a daring ruse that afforded Janina what she described as her “alter ego”.

Her considerable skills lay in methodological planning and in subverting often dire circumstances to her own ends. By becoming a member of the Catholic nobility, adapting her dress and manner to the style of a “lady bountiful” do-gooder, Janina flattered Nazi snobbery in offering a fake respectability to their barbarism. From that followed her services to “assist” the Reich in the smooth running of three extermination camps, first in her native Lvov, then in Lublin and Majdanek, from 1941 until the War’s end.

She presented successive Nazi Commandants with solutions to problems they had failed to address. Politely but insistently, she pointed out that to maximise productivity in slave labour camps, a minimum amount of food needed to be provided. It also followed that if the German Army was to maximise its operational efficiency, it required healthy soldiers. To such axiomatic but apparently previously unconsidered issues, she offered a logical solution through the welfare organisation she founded and directed. Taking responsibility for supplying food and medicines in requisite quantities, she ensured the war machine ran with maximum effectiveness. It became the ideal front for her life-saving work as an officer with the Polish Underground.

Within a year Janina had developed means to conceal messages sent by family, friends and Partisan colleagues, most often in the removable base of the soup churns delivered daily. She also insisted thatdoctors treat prisoners often suffering from highly infectious diseases, including typhus and tuberculosis, involving the Red Cross. Her technique for facing down suspicious Camp Commandants was straightforward. Pulling herself up to her smartly dressed full five feet tall, she concealed the terror that her Jewish identity might be on the point of being discovered, telling herself to “lock fear in your heart and perform your part”.

Her moral armoury possessed a further weapon. Above all, she relied on and imparted hope. Mass murder on an unprecedented scale — 6 million citizens killed, 18 per cent of the pre-War Polish population — meant mass desperation, often despair. Hope meant defiance. Janina’s mission was to assist her people in staying alive until liberation came.

After the war, Janina declined to discuss her wartime work. She emigrated with Henryk to Canada, then to the United States, where they both pursued successful academic careers.

So matters remained until 1989, 20 years after Janina’s death. Then, following a conference lecture, Holocaust historian Dr. Elizabeth White was handed Janina’s brief unpublished memoir. Four years of research in collaboration with fellow academic Dr. Joanna Sliwa followed to create the biography Janina richly deserves. It greatly honours an unsung true wartime heroine in her multiple roles as relief services co-coordinator and courier and spy for the Polish Underground, not to a brilliant woman of phenomenal moral courage.

‘The Counterfeit Countess’ by Elizabeth B White and Joanna Sliwa

Simon and Schuster, £18.99