The extraordinary story of two British sisters who became unlikely heroines in helping Jews to flee Nazi persecution is to be told on the big screen.

Ida and Louise Cook were two unassuming civil service secretaries whose passion for opera became their pretext for travelling repeatedly to Germany in the 1930s. While they toured the country’s opera houses, they also secured a safe passage for dozens of people who would otherwise have perished in the Holocaust.

Now a major feature film, The Cooks, is being produced by Donald Rosenfeld, former president of Merchant Ivory Productions, who made period classics such as Howards End, starring Emma Thompson.

Having worked on four productions with Thompson – with a fifth project already under way – Rosenfeld would now like to cast her as one of the Cook sisters alongside fellow Oscar-winner Cate Blanchett.

He paid tribute to the Cook sisters’ bravery. They were mingling with high-ranking Nazis at the opera, while helping people flee. Recalling the musical about the Trapp family’s escape from the Nazis, he described the Cooks’ story as “the real Sound of Music”.

The problem is that, if this was a fictional film, it would be unbelievable. How did two secretaries on meagre salaries fund their visits to Germany? In Ida’s 1950 memoir, which gave little away, she wrote of saving up their pennies to make their first trip to the opera.

The film-makers suspect that the sisters were also spies for the British government. Their research has led to the discovery of official files held by the CIA, which are completely sealed. They hope that two senators and a lawyer can get them opened.

“The fact that there are files in the CIA means that there are probably files in MI6,” Rosenfeld said. “It’s an incredible story of these two sisters who basically lived together. They never had husbands or children. They provide incredible roles for two actresses.”

Born in Sunderland, the sisters shared a modest terraced home in Wandsworth, south London. Coming to music late, after Louise happened to hear some Puccini, they funded their German trips partly with money that Ida began to make from writing romance novels for Mills & Boon, under the pseudonym Mary Burchell.

That still does not explain how they could afford repeated flights, Rosenfeld said: “You didn’t have an air service throughout Europe. They’re taking these flights into a dictatorship which is about to invade France. They’re doing it for years, going to different cities where the opera is taking place. The expense of that alone would be insane. There’s no way they could afford it.”

Even the cost of opera tickets would have been huge, he added: “There was no internet then to pre-buy them. There are so many impossibilities. It seems like an absolute operation.”

Between 1937 and the outbreak of war, the Cooks aided the escape of dozens of people, including Lisa Basch, the daughter of German-Austrian intellectuals, who said that the sisters saved her from the gas chamber.

Somehow, the Cooks secured the financial guarantees then needed for Jewish refugees to come to Britain. They were not allowed to leave Germany with their possessions, so the sisters smuggled them across borders. They wore clothes bought at Woolworth’s and looked so ordinary that customs officials never suspected their accompanying diamonds and other jewels were anything other than paste.

Isabel Vincent, an investigative reporter, is now delving into their story in writing the screenplay and a book. She believes that the sisters received support from the highest level. These intrepid women – both in their 30s – could have picked up vital intelligence. They were staying in the finest hotels, where they saw Hermann Goering, Joseph Goebbels and Heinrich Himmler.

Among files held at Blythe House in London, Vincent saw opera programmes that the sisters saved, including one from Munich in 1938 with Nazi insignia and an introduction by Adolf Hitler.

Another fascinating character is the conductor Clemens Krauss. Vincent said: “He’s an Austrian who goes to Berlin and joins the Nazi party. He’s some kind of double agent. [He] was setting up programmes in cities where the sisters needed to help people. So it’s an operation.”

She is struck by the many holes in Ida’s memoir: “There’s maybe two chapters about a few people that they helped, and it’s kind of glossed over.”

On one level, the sisters were the “ultimate groupies”, queuing for singers’ autographs. The singers appreciated their passion, inviting the sisters to their homes. Vincent has a letter from soprano Amelita Galli-Curci thanking them “for the beautiful handkerchief that you embroidered for me”.

In fleshing out the sisters’ true story, Vincent is now trying to identify people they saved. In Denver, she found a now-deceased musician from the Vienna Philharmonic, who had written to Ida for years. His daughter is sharing the letters with Vincent.

The film-makers have about 27 possible addresses in and around London for other people. Rosenfeld is appealing for anyone with a connection to the Cooks to contact Sovereign Films, his production company.

Ida and Louise died in 1986 and 1991 respectively. In 2010 they were posthumously honoured by the government as British heroes of the Holocaust.

“They are huge heroes,” Rosenfeld said. “This movie is an opportunity to make a global tribute to them.”