Poland's defence minister accused European Council President Donald Tusk of working with Russia's Vladimir Putin to harm Polish interests in connection with a 2010 plane crash that killed President Lech Kaczynski and 95 others in Russia.
The ministry notified the military department of the National Prosecutor's Office on Monday that it suspected Tusk, who was prime minister at the time, of an "abuse of trust in foreign relations".
A spokeswoman for the prosecutor's office confirmed the wording, which amounts to an accusation of diplomatic treason.
READ: Polish president and elite killed in plane crash
Defence Minister Antoni Macierewicz told the Gazeta Polska Codziennie daily on Tuesday: "Tusk made an illegal contract with Vladimir Putin to the detriment of Poland and should bear criminal responsibility for that."
The notification covers the period from April 10, 2010, when the plane crash happened in Russia's western city of Smolensk, to 2014, when Tusk took up his current post, in which he chairs summits of EU leaders.
Macierewicz alleged that before the crash, a contract to carry out maintenance in Russia on the Tu-154 plane was arranged by Tusk to be given to "a friend" of Putin.
"It's not about negligence, it is about a criminal offence," the minister said.
Tusk has frequently denied any responsibility for the crash, which an earlier official investigation concluded was an accident. His office in Brussels had no immediate comment.
The accusation marks a sharp escalation of the conflict between Poland's right-wing Law and Justice (PiS) party, led by Kaczynski's brother Jaroslaw, and Tusk, who led the rival Civic Platform party and was prime minister from 2007 to 2014.
The Polish government tried and failed earlier this month to block Tusk's reappointment to a second term in his EU post and replace him with an alternative Polish candidate.
The alleged crime carries a sentence of one to 10 years in prison. The prosecutor's Office is instructed, but not obliged, to decide whether to start an investigation within 30 days of receiving the notification, spokeswoman Ewa Bialik said.
Polish prosecutors are already conducting several investigations into the Smolensk crash, including a case against a group of public officials also suspected of acting to Poland's detriment in the year following the accident.
Russia has repeatedly refused Poland's demand to return the Tu-154 wreckage and its black box recorders, citing its own ongoing investigation.