The University of Parma hospital said the 61-year-old had been brought in after suffering from vomiting and dizziness and was undergoing surgery.
Bersani was given the chance in March to try to form a new Italian government after his coalition failed to win an outright majority, creating a political stalemate.
He stepped down as head of Italy's Democratic Party (PD) after failing to form a government, and fellow leftist Enrico Letta was appointed premier.
Messages of support flooded in on Sunday from across the political spectrum. Letta was reportedly in telephone contact with Bersani's family and even his long-time rival from the centre-right, Silvio Berlusconi, sent "an affectionate hug to a loyal adversary".