WARSAW (Reuters) - A Polish parliamentary inquiry plans to question European Council President Donald Tusk over an investment scam in which thousands of people lost money while he was prime minister, the head of the inquiry committee said.

The announcement follows comments last month from Jaroslaw Kaczynski, head of the ruling eurosceptic Law and Justice (PiS) party, that Tusk might face charges in Poland.

The Amber Gold Ponzi scheme - a type of fraud that relies on paying initial investors with money received from later ones - ran for over two years before collapsing in 2012. It resulted in hundreds of millions of zlotys in losses for over 10,000 depositors.

Tusk, a former head of the center-right, pro-European Civic Platform party (PO), served as prime minister for about seven years before resigning in 2014 to take the top European Union job. PO was ousted in last year's elections by PiS, and Kaczynski wants to block his old opponent from being awarded a second term next year as chair of the EU's summit meetings.

Asked if Tusk would be invited to testify in parliament, Malgorzata Wassermann, a PiS lawmaker who heads the inquiry committee, told state television late on Monday: "This is rather a done deal, this is obvious."

Amber Gold promised fixed returns often exceeding 10 percent a year from an alleged purchase of gold, while at the same time pouring millions into a low-cost airline. It opened outlets in prestigious locations across Poland, including near the central bank and finance ministry.

The company posted a huge billboard in central Warsaw advertising "a deposit in gold", despite an earlier warning to potential clients issued by the financial supervisor.

A state report published in 2013 said Amber Gold's expansion was possible due to "mistakes and non-action" by some state institutions, which did not perform their duties properly.

Wassermann said that Tusk would most likely testify before the parliamentary panel at the end of its proceedings to answer questions related to his indirect oversight of law enforcement and secret services through cabinet ministers.

Tusk told Polish media in July that he was ready to confront PiS and defend his record. "I would like to come and say, you are lying, the truth looks completely different," he said.

(Reporting by Marcin Goettig)