Israel will on Thursday honour the Polish couple who hid Oscar-winning director Roman Polanski from the Nazis as a child, the Holocaust memorial Yad Vashem said.

The late Stefania and Jan Buchala will receive the Yad Vashem title of "Righteous Among the Nations" for those who helped save Jews during World War II.

"Despite their very difficult economic situation, the couple agreed to take in the Jewish boy as their own son, and keep him safe," Yad Vashem said.

Their grandson will accept the medal at the ceremony in southern Poland that has been kept under wraps for security reasons and which local media say Polanski himself could attend.

The controversial film director was just six years old when Nazi Germany invaded Poland in 1939, triggering the war and forcing the family into the Krakow Ghetto.

After his father smuggled him out through the barbed wire and got friends to take him in, Polanski was shuffled around, then sent to the Buchalas.

Devout Catholic peasants with three small children, they gave him shelter for nearly two years in the village of Wysoka, asking for nothing in return.

The now 87-year-old French-Polish director of "Chinatown" and "Rosemary's Baby" recalled their "astounding" kindness in his autobiography, calling Stefania "strong", "energetic" and "sensitive".

Polanski had tried for years to track down the family to thank them -- to no avail. Both Buchalas died in 1953 and the village had no news of the children.

It was only in 2017 that Polanski came face to face with their grandson thanks to some detective work from the filmmakers behind "Polanski, Horowitz".

Due out next year, the documentary recounts the childhood years of Polanski and his lifelong friend and fellow Holocaust survivor, photographer Ryszard Horowitz.

Producer Anna Kokoszka-Romer recounted the first meeting between Stanislaw Buchala and Polanski when the grandson showed the director old photos.

Roman Polanski was just six years old when Nazi Germany invaded Poland
Roman Polanski was just six years old when Nazi Germany invaded Poland AFP / LOIC VENANCE

"Polanski was moved. As for Stanislaw Buchala, you could see his joy at finally getting to learn more about his family," she told AFP.

His late father Ludwik had talked about growing up with a "brother" who moved to the US after the war. Little did he know that his childhood playmate became filmmaker Roman Polanski.

Stanislaw, who is in his 60s, told the filmmakers he was proud of his "heroic" grandparents.

"People should know, especially now during the pandemic, that a person can do something selfless for another."

In his testimony to Yad Vashem, Polanski said Stefania Buchala "provided me with shelter, risking her own life and that of her family."

The stakes were high. In occupied Poland, even offering Jews a glass of water was punishable by death.

Poland, which lost six million citizens -- half of them Jewish -- during the war, has the most "Righteous Among the Nations" of any country, at more than 7,000 individuals.

The filmmaker lives in France, as he is persona non grata in Hollywood and cannot return to the United States for fear of arrest.

Polanski pleaded guilty to unlawful sex with a minor in California in 1977, in a plea deal after he was accused of raping a 13-year-old girl. He fled the US before sentencing.

The director of Yad Vashem's Righteous Among the Nations department, Joel Zisenwine, said "the award is given to the rescuers, not the survivor".

"It is about what they did in real time, risking their lives to save a nine-year-old child," he told AFP.

"Nobody of course has any way of knowing what happens to people as adults. It has nothing to do with that."