The 86-year-old billionaire media tycoon, who has suffered repeated bouts of ill health in recent years, had come out of hospital last week.
Berlusconi's Forza Italia party is part of Prime Minister Giorgia Meloni's right-wing coalition although he does not have a role in government.
Berlusconi, who made his fortune through his television channels before entering politics, had heart surgery in 2016 and has also had prostate cancer.
He has been repeatedly admitted to hospital in the past couple of years after contracting COVID-19 in 2020.
Three sources from Berlusconi's Forza Italia party said he was in intensive care, and one of them confirmed Italian media reports that he was being treated in a cardiac unit.
Another of the sources said the situation was "under control".
San Raffaele Hospital did not immediately respond to a request for comment.
Berlusconi, who sits in the Senate (upper house) of the Italian parliament, has stirred controversy in recent months with his criticism of Ukraine's President Volodymir Zelenskiy, putting him at odds with Meloni.
An Italian court acquitted Berlusconi in February over allegations of paying witnesses to lie in an underage prostitution case that has dogged the former prime minister for more than a decade.
Berlusconi was accused of bribing 24 people, mostly young, female guests at his so-called Bunga Bunga parties, in a previous trial where he was charged with paying for sex with a 17-year-old Moroccan nightclub dancer.
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