WARSAW (Reuters) - The head of Poland’s ruling party, Jaroslaw Kaczynski, launched an attack on Tuesday on European Council President Donald Tusk, an old political foe at home, saying he should not be allowed to serve a second term.

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Tusk, a former head of the center-right, pro-European Civic Platform party, served as Poland’s prime minister for almost two terms before resigning in 2014 to take the top European Union job.

His mandate as council president, in which he chairs summits of EU leaders, expires at the end of May 2017, although he is expected to seek to stay on in the post.

Kaczynski, head of Poland’s nationalist-minded Law and Justice (PiS) party, said in a newspaper interview that the current PiS government would not support Tusk staying on.

That was because Tusk might find himself facing charges, Kaczynski said - apparently referring to his failure as prime minister to stop the Ponzi scheme Amber Gold which shook the country in 2012. PiS dominates a parliamentary panel set up to investigate the case.

Despite Kaczynski’s words, a PiS government cannot stop Tusk renewing his mandate as long as a majority of EU leaders back him, which seems likely.

Most appear satisfied with Tusk’s performance and are keen to avoid more instability as the EU starts negotiations over Britain’s decision to leave the bloc.

But the comments highlight the conflict between the Polish rivals that could complicate Tusk’s candidacy.

“The Polish government will not support Donald Tusk for the second term in the European Council,” Kaczynski told the Polska The Times newspaper in an interview published on Tuesday. “Tusk is a major problem.”

“There are investigations going on in Poland in the parliament and in the prosecutors’ office that may lead to pressing some charges against him. Should such a person be the head of the European Council? I have serious doubts.”

“His further stay in Brussels is highly risky, mainly for the European Union,” Kaczynski told the paper.

Tusk responded on Twitter by challenging Kaczynski to a public debate “on Europe, Poland and your insinuations”.

Kaczynski believes Tusk shares some responsibility for a 2010 plane crash in Russia in which his twin brother and then-president Lech Kaczynski was killed along with 95 others. Tusk was prime minister at the time.