Ukraine's former leader Petro Poroshenko has been designated a witness in a criminal investigation related to the nomination of judges, the state investigation bureau said on Monday.
Poroshenko has been embroiled in a number of investigations since leaving office in May.
But the latest investigation is the first abuse of power probe involving the former president, his office said.
"His status is that of a witness," a spokeswoman for the bureau, which handles high-profile cases, told AFP.
If Poroshenko's status changed to that of suspect, he could be prosecuted if parliament voted to lift his immunity as a lawmaker.
If that happened, and he was convicted of abuse of power and calls to overthrow the government, he could face up to six years in prison.
Poroshenko's office denounced the bureau's announcement as an attack sponsored by arch-enemy Moscow to "damage" his reputation.
According to a statement released by the state investigation bureau earlier in the day, the 54-year-old former president is suspected of trying to "illegally influence" the makeup of the High Council of Justice earlier this year.
The council is a state body which picks judges who are then appointed by the president.
Ukraine's president between 2014 and 2019, Poroshenko was routed by Volodymyr Zelensky, a 41-year-old comedian with no previous political experience, in an election in April.
He is now a lawmaker in a parliament dominated by Zelensky's ruling party.
The head of the state investigation bureau, Roman Truba, has said that Poroshenko, a wealthy businessman, has been named as a witness in a dozen criminal investigations including a probe into alleged tax fraud.
The probes were opened following allegations of a former senior official, Andriy Portnov, Poroshenko's foe who lived in Russia and Austria during his presidency.
Portnov is a close friend of Andriy Bodgan, head of Zelensky's administration.