The probe into possible abuse of office concerns 35 plane and helicopter flights Salvini took when he was interior minister in the previous government, according to Italian media.
The prosecutor has asked the court to determine whether the flights on police and firefighters’ planes and helicopters constituted an abuse of office.
According to a recent investigation by La Repubblica, Salvini often scheduled official state trips within Italy just before or after rallies for the League.
Ministers can use state flights for institutional reasons but not for campaigning, which Salvini is suspected of doing as he travelled around Italy for months, sometimes combining interior ministry functions with party rallies.
“I read that I am under investigation,” Salvini said on Thursday, after several newspapers carried reports of the investigation.
“All my state flights were for state reasons... I never took state flights to go on holiday, that’s what other politicians do.”
Italy’s audit court had earlier opened an investigation, but it was shelved in September on the grounds that the illegal flights did not come at any extra cost to the taxpayer.
However, the court still considered the flights illegal because Italian law stipulates that police and firefighting aircraft should be used exclusively for institutional use and not by state ministers.
Only the five highest members of state – the president, the two parliament speakers, the prime minister and the head of the constitutional court – are legally allowed to use such aircraft.
Exceptions most be specially authorised, which was not the case for Salvini.
Legal problems have dogged Salvini and the League for months.
In February, while Salvini was still a minister, parliament blocked an investigation into accusations of kidnapping over his decision to hold 150 migrants on board a ship for five days in August 2018.
Police searched offices of the Lombardy regional government and firms linked to the League on Tuesday in a money laundering probe connected with a 2018 ruling that ordered the League to repay €49 million it owed the state.
Prosecutors also opened an investigation into possible international corruption following media reports that the League tried to raise millions of euros via a secret Russian oil deal.