The 20-year-old midfielder has caught the eye of a host of European clubs after a standout season for Brescia last term, with Inter in lengthy negotiations for his signature.
However, it appears that Milan have won the race to snap him up after Inter turned their attention elsewhere, and Sport Mediaset report that he will undergo a medical on Thursday before penning a five-year contract the following day.
Milan have agreed an initial season-long loan with the option to make the transfer permanent for €30 million, but the move is dependent on Tonali signing a contract extension with Brescia first.
With no guarantee that Milan will take up their option next year, Rondinelle president Massimo Cellino is determined to protect his asset should Tonali return to the club in 2021, as his contract would expire the following summer.
The Italy international is expected to extend his existing deal with Brescia before completing his switch to Milan later in the week.