German cake and biscuit producer Bahlsen relied heavily on forced labor during the Nazi era and it benefited from close ties to the Nazi regime, the results of a historical investigation into the Hanover-based company found.

Business for Bahlsen, best known as the makers of classic Leibniz biscuits, Choco Leibniz and Pick Up chocolate sandwich biscuits, increased enormously during the Nazi years in Germany, according to historians Manfred Grieger and Hartmut Berghoff.

They found that the 135-year-old company used more than 800 forced laborers between 1940 and 1945, many of them women from Ukraine and Poland. Those laborers were required to wear patches on their clothing that identified them as foreigners and subjected them to intense discrimination.

The historical research into the company's past came after the heiress to the company fortune, Verena Bahlsen, claimed in 2019 that forced laborers at the company were "treated well" during the Nazi era.

Her remarks prompted a storm of outrage and condemnation in Germany, and she issued an apology that described her comments as a mistake.

The family-owned company commissioned and funded the historical investigation.

"We were not aware of many details of the company's history, and the truth is that we did not ask," the Bahlsen family said in a statement before the book's publication.

The research has now resulted in a 600-page book detailing the company's history from 1911 to 1974, which went on sale on Wednesday.

The authors wrote that the company did not place any conditions on the content of their work and said they remained in close contact with both Verena Bahlsen and her father, Werner M. Bahlsen.

The historians found that Polish forced laborers at Bahlsen received lower wages, smaller food rations and poorer medical care than other workers and were housed in barracks and excluded from public life.

The truth about the events of that time was uncomfortable and painful, the family added in the statement.

"We deeply regret the injustice that happened to these people at Bahlsen. We also regret that we did not face up to this difficult truth sooner."

For decades, the company had sought to downplay memories of forced labor and describe the war years as a period of amicable cooperation under challenging times. Now, the company has acknowledged its true history.

"It is up to us to keep this memory alive and ensure that this history of National Socialism is never repeated," reads a commemorative plaque in the foyer of the company headquarters.