The reported £55m fee would make him the most expensive Italian footballer of all time.

Tonali helped Milan win Serie A in 2021-22 and was part of the side which reached last season's Champions League semi-finals.

He was also a member of Italy's squad at the Under-21 European Championships.

Tonali told the club website: "I want to repay the trust on the pitch, giving it my all, as I always have. I'm really excited about playing at St. James' Park, I can't wait to feel the warmth of the fans."

Newcastle head coach Howe said of Tonali: "He is an exceptional talent and has the mentality, physicality and technical attributes to be a great fit for us.

"At 23, Sandro already has important experience as a key player in one of Europe's top leagues and in the Champions League, as well as playing for his country.

"But he also has the opportunity and potential to grow and evolve with us, and I'm excited to add him to our squad as we approach the exciting season ahead."

Tonali came through the ranks at Brescia and made his professional debut for them in Serie B at 17.

Brescia were promoted to Serie A in 2018-19 and, though the club was relegated the following season, Tonali made his Italy debut during that campaign when he came on as a substitute against Liechtenstein in October 2019.

He joined AC Milan on loan in the summer of 2020 and, after making the move permanent the following year, scored five goals in 36 top-flight appearances as the club won the Italian title in 2022.

Tonali scored twice in 34 Serie A outings as the San Siro side finished fourth last season, while he made 12 appearances in the Champions League before Milan were knocked out by rivals Inter in the semi-finals.

Analysis

Harry De Cosemo, BBC Sport

Eddie Howe stressed at the end of last season that this summer would be Newcastle's toughest transfer window yet.Champions League qualification has increased the need for quality additions, but after three windows in which the club's owners have spent without recouping much money, Financial Fair Play has become a more prudent consideration.

Tonali can play both deep and more advanced in the middle of the park, meaning he will relieve the creative burden on Bruno Guimaraes at crucial times during the season. Towards the end of the last campaign, Guimaraes' performances were hampered by an ongoing ankle problem.The Italian didn't feature heavily on initial lists of targets, but Newcastle were encouraged to push for the deal when they realised the 23-year-old was available within their budget, deciding then to move away from an initial plan to sign a specialist holding midfielder.

Versatility and athleticism are two of the most crucial factors Eddie Howe is looking for in his summer signings, and Tonali certainly offers both. He will bring another dimension to their midfield.