The week of Holocaust Memorial Day has always been an emotional one for me. The day was chosen to mark the liberation of Auschwitz, but it also coincides with the week in which my mother was released from Bergen-Belsen and, therefore, with the death of my grandmother. And then, three years ago and remarkably in the same week, my Mum died too.

When the British arrived in Belsen they encountered an horrific scene. Piles of dead bodies, thousands upon thousands of sick and dying people, a smell of decay and decomposition beyond description. I have had many messages from people who numbered among their family those who liberated the camp. Many say their relatives never spoke about it, and that sometimes, decades later they would happen upon their dad or uncle or whoever and find them crying.

To each of these correspondents I make sure to send a message of thanks from our family for what they did and the burden they as families carry.

When the end came in Belsen my mother was no longer there. From the summer of 1944 the camp had begun to collapse. It did not have the facilities to cope with thousands of new inmates brought from elsewhere as the Germans retreated and sought to cover up their crimes. There was no water, hardly any food (even less than the miserable amount given out before), inadequate toilet facilities. Disease spread, starvation became almost universal. Belsen was an extermination camp at the end, just with different means of eradication.

So the chance to leave was a life saving chance. And it fell to my mum, her sisters and my grandmother because they had a Paraguayan passport. This document was almost certainly responsible for them being sent to Belsen rather than to the gas chambers. And then, by luck, it saw them selected for an exchange. Himmler had wanted to do many thousands of exchanges, for Germans living abroad, for money and for equipment. His terms were so unrealistic that in the end only a few hundred ever made it. And Mum was one.

But I have always wondered what on earth that Paraguayan passport was. It states that Margarethe Wiener, my grandmother, came from Paraguay, which she most certainly did not. As far as I know there is and never has been any link at all between our family and Paraguay.

This week I found out. I did know the passport was obtained in Bern in Switzerland by a friend of my grandfather’s. What I did not know, because it was not generally known, was that it came from a brave and successful effort by a group of Poles to save thousands of Jews.

The Bern embassy was occupied by the Polish government in exile under the ambassadorship of Aleksander Lados. A group working for him and with an exiled Polish politician and Zionist called Abraham Silberschein began to buy blank Paraguayan (and other South American) passports and fill them out with the details of Jews, claiming falsely that they came from Paraguay.

My mother’s passport is signed by a man called Rudolf Hügli. He was Paraguay’s honorary counsel in Bern and sold the blanks and then signed them once they were filled out. He was, apparently, well paid. Silberschein and Chaim Eiss provided the finance.

The writing on the passport — quite distinctive — is, the Polish embassy in Switzerland informs me, that of Konstanty Rockicki one of the embassy team. Juliusz Kuhl and Lados’s deputy Stefan Ryniewicz made up the rest of what is called the Lados group.

Buried behind the Iron Curtain little has been known about this extraordinary effort. This is now going to change. A Lados list has been assembled with thousands of beneficiaries on it, many of them survivors of the war. A documentary has been made, and there are books to come.

I have plenty to learn still. What was the exchange in Mum’s case for instance? But I do know this. Without the Lados Group my mother would not have survived. She would have gone to Auschwitz or Sobibor. Or she would have died of disease and hunger in the last moths of Belsen. As it was my grandmother starved to death on the night of freedom. But these heroes saved her little girls.

Daniel Finkelstein is associate editor of The Times.