Donald Tusk has been summoned for questioning in Poland over the 2010 plane crash that killed President Lech Kaczynski, prosecutors said.
It was not immediately clear whether Mr Tusk, freshly re-elected as European Council chief, would come for the questioning.
He has been summoned as a witness.
The leader of Poland's ruling populist party, Jaroslaw Kaczynski, blames his brother's death on Mr Tusk and has been taking every step to bring him to account.
He says Mr Tusk, a former Polish prime minister, neglected the president's security and then left the investigation of the crash in Russia's hands.
Mr Kaczynski said on Monday he had "nothing to do" with the summons, but stressed that Mr Tusk's name is mentioned in many investigations concerning the time when he ran the government from 2007-2014.
He conceded, however, that efforts last week by Poland's government to block Mr Tusk's re-election as EU Council head in Brussels had a "personal aspect".
Michal Dziekanski, spokesman for Warsaw prosecutors, said that Mr Tusk has been summoned for questioning on Wednesday.
The case concerns co-operation in security that Poland's military intelligence agreed on with Russia's security services following the 2010 plane crash in Russia that killed Poland's president.