He has suffered death threats and is boycotted by the Polish community in Canada, where he lives today. Even in his homeland he is not always welcome, but Polish historian Jan Grabowski, 50, does not give up. He is determined to continue his struggle to expose the truth.

The books published by Mr Grabowski over the past few years have one theme in common: Polish participation in the killings of their Jewish neighbours.

His most recent, The hunt for Jews: betrayal and murder in Poland under German occupation, is no exception, and was published this month in English.

In the book, Mr Grabowski, who is not Jewish, describes several massacres in which Poles volunteered to help the Nazi exterminate the Jews.

“The original book in Polish is called Judenjagd — The hunt for the Jews. The English book is an improved and extended version. It includes many new discoveries, pieces of evidence and cases not previously published.

The book describes massacres that Poles helped carry out

“For some in the Polish society my previous books were dirty laundry that should have not been taken out. There were a few Polish newspapers with nationalist views… for whom the purpose of my book was only to tarnish the image of Poland,” said Mr Grabowski.

In the three years he took to research and write the book, the author visited several countries to interview survivors and local residents, especially in Poland, Israel and Germany. Many of the massacres on Polish soil took place in small villages, Mr Grabowski said.

Among other things, he studied the verdicts of dozens of post-war trials of Polish residents who were tried by the Communist regime for taking part in the killing of their Jewish neighbours.

“After I published the book in Polish, I was contacted by Holocaust survivors. They recounted some amazing stories that appear in the book.

“The most interesting change in the English version is that it includes two diaries written by Jews while they were hiding in Poland, as well as new oral testimonies by Polish peasants and Jewish survivors who hid and managed to stay alive,” said Mr Grabowski.

The Hunt for Jews corroborates the evidence put forward in Golden Harvest, a book published in 2011 by Princeton University professor Jan Gross, which also investigates Polish collusion in the Holocaust.