Amid the barbed wire, gas chambers and shooting pits of the horrific Majdanek concentration camp in Nazi-occupied Poland, Countess Janina Suchodolska was a surprising yet frequent visitor in 1942. A pretty brunette with blue-green eyes, an aristocratic air and the authority derived from generations of nobility, she made demands of the camp’s commandant and guards, SS officers and Gestapo as if she owned the place.

She brought food, medicine and hope to the camp that held 23,000 starving prisoners dying of disease with no running water, open latrines and contaminated wells showered with ashes from the crematorium burning the relentless flow of bodies.

Barely 5ft 1in tall, she risked her life standing up to Nazi murderers, while secretly ferrying messages and supplies to imprisoned members of the Polish resistance, and smuggling in tools to aid escapes.

Yet it was all a dangerous act.

“She was not a countess at all,” reveals Elizabeth White, co-author of the gripping new book The Counterfeit Countess. “She was unique: a Jew who saved thousands of non-Jews from the Nazis.”

Oskar Schindler famously saved 1,200 of his Jewish factory workers from the Holocaust, immortalised in Steven Spielberg’s 1993 film Schindler’s List, but the numbers he rescued were dwarfed by those saved by the woman claiming to be a countess.

“It’s one of the most remarkable, selfless acts of heroism of the Second World War, yet astonishingly her story has never been told before,” says the book’s co-author, Polish Holocaust expert Joanna Sliwa.

“Her real name was Janina Spinner Mehlberg, a brilliant mathematician, an officer in the underground Polish Home Army, and a Jew. She was a charismatic woman who hid her fear as she confronted the Nazis.”

Born in 1905, she worked as a maths lecturer in Lvov until she and her husband, who taught philosophy, managed to escape the Jewish ghetto following the Nazi invasion and occupation. Assuming non-Jewish identities in Lublin, they lay low.

But when Jews, peasants and political prisoners began disappearing into Majdanek, and the smoke from hundreds of cremations began to stain the sky, her conscience could not settle for mere survival.

Her life would have been of no value unless she helped others. Her aim, as her ­husband put it, was “not to live uselessly, nor to die pointlessly”.

Sliwa continues: “She saved the lives of 9,707 Poles, plus many more who survived the war thanks to her keeping them alive with food and medical supplies. Yet no matter how many people she saved, she never thought she was doing enough. She believed her life was worthless if she wasn’t saving others.”

Risking her life daily, Janina posed as an aristocrat, which in itself helped open doors, while working for the Polish Main Welfare Council relief group, and demanded improved conditions while she spied on the Majdanek concentration camp.

“She knew she would be gruesomely tortured and killed if the SS discovered her smuggling activities or her true identity,” says White, a retired Holocaust crimes investigator for the US Justice Department. “But she felt that her life would have no meaning if she didn’t act to save others.”

The town of Lublin, where at least 63,000 Jews were slaughtered in Majdanek’s gas chambers or shot in large numbers in trenches, was the centre of the Nazis’ largest mass murder operation of the Holocaust.

“Majdanek was a hellhole in 1943, with the highest mortality rate of any Nazi camp,” White explains. “Auschwitz ran a distant second.”In Janina’s unpublished notes, discovered long after the war, she wrote: “It would have been natural to give up, to stop going to Majdanek, to go to pieces, even. But then there would have been no reason to live.

“When so many were in such terrible need, I had to live to answer that need… If I thought only of the dangers to myself or to those I loved, I was worth nothing.”

Her husband Henry wrote: “She might die, and many times knew this might be the moment, but not for nothing. Not to live uselessly, nor to die pointlessly.”

The fake countess stood up to senior Nazi officials, pressuring them for prisoners’ release, or their better care.

“She was often afraid, but never let it show, staring Nazi murderers in the eye and maintaining incredible self-control,” says White. “She refused to take ‘no’ for an answer. If she was denied by one official, she would go over their head to a more senior officer.

“She never flinched when the SS shouted in her face, and the prisoners marvelled at her success in winning astonishing concessions from Nazi officials.”

Janina alerted health officials to a typhus epidemic among the prisoners, forcing the camp commandant to reluctantly provide treatment. And after the Germans imprisoned 3,600 displaced Polish peasants, she demanded their release when they soon began dying of starvation, dehydration and disease.

Yet having secured their freedom, hundreds were too weak to walk away.

Civilian vehicles were barred from the camp, but the countess ordered an armada of trucks to pick them up anyway, saving the 2,106 surviving peasants released in August 1943. “Janina felt she had failed in not saving more of them,” says White.

The phoney countess relentlessly harassed senior Nazis for more deliveries of life-saving supplies to prisoners, even bringing decorated Christmas trees and Easter eggs to boost morale. Her welfare mission grew, until she brought tons of bread and hundreds of gallons of soup daily – a programme unique among Nazi death camps.

With the deliveries, she smuggled in messages and equipment for the resistance. “She persuaded the Germans that it was in their best interests to keep the prisoners alive, so that they could do the hard labour required,” Sliwa explains.

In addition to her work at the concentration camp, Janina rescued children taken by Nazis from their families, as well as Poles seized for forced labour in Germany. Increasingly suspicious that she was aiding the resistance, the Gestapo followed Janina and sent spies in an attempt to entrap her.

“There were many times when Janina was almost discovered or denounced, only narrowly escaping arrest, torture and death,” says Sliwa. “Yet she persisted.”

Born in 1905 as Pepi Spinner, the daughter of a wealthy landowner in Lvov, Poland – now Lviv in Ukraine – she was raised by a governess and tutors, enjoying an aristocratic lifestyle that would later save her life when she posed as the countess.

She earned a doctorate in philosophy at the age of 22 and worked as a maths teacher until 1941, when she fled with husband Henry to Lublin. An old family friend, Count Andrzej Skrzyski, helped them obtain fake identity papers as the Count and Countess Suchodolska, and also found Janina work with the Polish welfare agency.

Yet Janina was powerless to help Poland’s Jews, who were trapped in squalid ghettos or starved and gassed to death in camps, as her work with the welfare group only allowed her to aid Polish non-Jews.

“She had no access to Jews in Majdanek, but hoped that by supplying food and medicine to the camp that some would get to them,” says White.

When the camp was liberated by Russia’s Red Army in 1944, it shocked the world, providing proof of the gas chambers in Nazi death camps. Yet Janina’s troubles were far from over. In Communist post-war Poland, she was seen as a collaborator.

“The Communists were eliminating the Polish resistance fighters and considered aristocrats Nazi collaborators, so Janina was in danger,” says Sliwa.

“Jews were also being persecuted by Poles after the war, so she had to maintain her false identity, calling herself Dr Suchodolska.”

Janina and her husband defected from Soviet-controlled Poland in 1950 and fled to Canada, before settling in America. She became a professor of mathematics at the Illinois Institute of Technology in Chicago, where she died in 1969, aged 64, taking her secret heroism with her to the grave – until now. Towards the end of her life, she wrote an untitled memoir but it has never been published.

“It’s an extraordinary tale of courage and selfless compassion,” says Sliwa today. “She never told her story, thinking no one would believe her. If we hadn’t corroborated all her courageous acts in wartime records, I’m not sure I’d have believed her either.”

The Counterfeit Countess: The untold story of the Jewish heroine who defied the Holocaust by Elizabeth B. White and Joanna Sliwa (John Blake, £18.99) is out now. Visit expressbookshop.com or call Express Bookshop on 020 3176 3832. Free UK P&P on orders over £25