As he surveyed the horrific conditions inside the ghetto, Simcha Rotem suddenly heard the cry of a young infant. He saw a young mother holding her baby, but as Rotem walked closer, he realised her life had already ebbed away.

“What do you do?” asks Rotem, the last survivor of the Warsaw Ghetto Uprising and still deeply affected by what he witnessed 70 years on.

His rhetorical question can never be answered, but one that partly explains why he enlisted in a secret group of Jewish survivors led by Abba Kovner, who were determined to take revenge on the Nazis.

Known as “Nakam” (“The Avengers”), in 1946 they devised an audacious plot to kill six million Germans by poisoning the country’s water supply, as well as assassinate thousands of SS officers.

Their extraordinary plans – which were ultimately foiled – are detailed in a series of audio recordings broadcast for the first time in a new Channel 4 documentary, Holocaust: The Revenge Plot.

The tapes, recorded in 1985 by Kovner, who was dying of cancer and his fellow surviving comrades, describe how the group attempted to infiltrate the waterworks of four German cities – Hamburg, Nuremberg, Frankfurt and Munich.

They also reveal the details of a second plot to poison the bread of 50,000 SS officers held at prisoner of war camps in Nuremberg and Munich, which may have partially succeeded.

Their plans were bankrolled, somewhat ironically, by buying £5 notes forged by Jewish prisoners in concentration camps, and selling them on the Italian black market.

Somewhat sensationally, the tapes implicate two future presidents of Israel, Chaim Weizmann and Efraim Katzir, in helping The Avengers to acquire their poison.

The discovery of the long-forgotten tapes was nothing short of “amazing”, according to Israeli director Avi Merkado-Ettedgui.

Having first learnt about The Avengers a few years ago, Merkado-Ettedgui wanted to research further into the story and discovered all of Abba Kovner’s belongings had been donated by his family to the Moreshet archive in Israel.

Trawling through the boxes of unprocessed items, he came across a solitary tape of the 1985 meeting.

Merkado-Ettedgui tells me: “I thought, wow, that’s amazing. Then I talked to the son of another group member, Pasha Reichmann, and we went through his things. We found more. It was quite a detective moment. I returned to the archive and eventually we found ten tapes.”

In total, the recordings provide eight hours of material that have never been heard before – not even by those who took part in the meeting.

Abba Kovner, leader of Nakam, who made a serious of recordings in 1985 shortly before his death detailing the group’s audacious plot

Merkado-Ettedgui adds: “In 1985, Abba Kovner was dying and people were beginning to poke around the story. They decided to put everything on record, for future generations, what really happened.

“The tapes are really well-organised, each speaker knew when they had to talk and some are reading from old notes and interviews. You could feel this was an important moment for them and they wanted it to be accurate.”

Given their intentions for making the tapes, I ask Merkado-Ettedgui if he believes their claims Weizmann and Katzir were involved in helping procure poison for the group.

“There is a claim that Weizmann wasn’t even in the country, but from everything I’ve learnt about Kovner, I don’t see any reason for him to lie. He really describes the meeting in detail.

“As for the involvement of Katzir and his brother, I believe these things are true also. It’s amazing that two past presidents were involved.

“But it’s important to state Kovner never told them about their plans for mass murder, only about procuring poison for murdering SS soldiers.”

Alongside the recordings, the film features testimony from the last surviving members of the group, including Rotem, as well as Auschwitz survivor Yehuda “Poldek” Maimon and Hasia Warshawski, who publicly speaks for the first time about her experiences.

Their perspectives provide a unique insight into why they joined The Avengers.

“Their wartime experience was hell. They went through very difficult times and this was one way to cope. What made them do it? Why were they different to other survivors?

“Poldek, who spent 22 months in Auschwitz, told me that when he closes his eyes every night, he sees his parents being lead to the crematorium.

“Kazik is still affected by the crying baby he couldn’t help. Their experiences made them feel so humiliated. It wasn’t so much about losing loved one as it was about what the Germans made them do.”

Yehuda “Poldeck” Maimon recalls his experiences in the documentary

While many were troubled by the morality of indiscriminately killing six million Germans, there were fewer qualms over assassinating SS officers.

“I know that most of them still wish they could have killed more. They felt it was different to kill those who were actually a part of the extermination of Jewish people. That was justice.”

Following Kovner’s arrest and two failed plots, The Avengers started work on their next plan: to begin again in Israel.

“All of them said it was hard to come here, to give up their feelings of revenge, but they now had hope, a new life. Israel changed everything for them.”

Holocaust: The Revenge Plot, airs on Holocaust Memorial Day, January 27, 9pm, on Channel 4.