In an unusual kind of pre-High Holy Days study seminar, 22 young Polish Catholics spent three weeks in Israel learning about their Jewish roots.

They rank among the estimated tens of thousands of "hidden Jews" across Eastern Europe - people whose families hid their Jewish identity and in some cases subsequently forgot the fact they had Jewish lineage. In many cases their families began concealing their Judaism to avoid persecution by the Nazis and the Communists. Some "hidden Jews" were handed by their parents to non-Jewish individuals or institutions during the Holocaust and grew up not knowing they were born Jewish.

In the past decade, spurred by greater openness in the post-Communist country and easy access to information about Judaism over the internet, growing numbers have sought to find out about their family's heritage.

During their seminar, which was conducted in Polish, the participants travelled across Israel, studied Hebrew and learned about Judaism and Israel.

The event was organised by a non-profit organisation called Shavei Israel - or, "Returners to Israel" - which has three full-time emissaries in Poland. The organisation's founder and chairman, Michael Freund, said that while some may choose to start living a Jewish lifestyle - a move which in most cases would necessitate conversion to Judaism as most are not halachically Jewish - this was not the aim of the seminar.

"Some of the participants are interested in living in Israel and converting, others are in a process of exploration, finding out about their identity and working out whether and how to integrate it in to their lives - the decision of course is entirely up to them," he said.

One participant, Katya Loksova, a 21-year-old student from Warsaw, believes she had Jewish ancestors several centuries ago. She became interested in Judaism during her undergraduate degree, and today feels a strong connection to Judaism and hopes to convert and live for a stint in Israel.

"There is the sense that something was missing in my life and now I have the religion," she said.

Mr Freund established Shavei Israel in 2002 out of a conviction that Jews should help "members of the extended Jewish family" in their search for roots. Twists of history removed them from the Jewish people, but now that Jews have the ability to reconnect with them, he considers doing so as a "religious, historical and moral obligation". A keen Zionist, he also sees it as beneficial for Israel and Jewry in challenges they face, such as the fights against antisemitism and the delegitimisation of Israel.

"The Jewish people is small and getting smaller. Israel does not have that many friends abroad. These decendants of Jews could serve as a reservoir of support for Israel and the diaspora."

Some will inevitably opt to convert, "strengthening the Jewish people demographically."

As well as working with Poland's "hidden Jews", Shavei Israel operates in India with the "Bnei Menashe", who claim to be descendants of a lost tribe of Israel; in Russia with people known as "Subbotniks", who keep certain Jewish practices; in China with descendants of the Jewish community of Kaifeng, and in Spain, Portugal and South America with "Marranos".

Shavei Israel has just sent a new emissary to Portugal. Rabbi Elisha Salas, a Chilean-born Israeli, will work with Marranos - or Conversos, as they are referred to. He is based in Belmonte, where some 140 Conversos have been converted to Judaism by the Israeli Chief Rabbinate in the past two decades.

In February, Rabbi Nissan Ben-Avraham became the organisation's emissary to Spain. More than six centuries after his ancestors were forcibly converted to Catholicism, he converted to Judaism and received semichah.

"His appointment represented the closing of a historical circle," said Mr Freund.