MILAN (AP) — Former Premier Silvio Berlusconi is suffering from a severe case aortic insufficiency and will undergo open-heart surgery to replace the valve next week, his doctor told reporters on Thursday.

The doctor, Alberto Zangrillo, said the 79-year-old billionaire businessman "risked dying," indicating a condition more severe than had been revealed by friends and family who have been visiting the former premier at Milan's San Raffaele clinic since he was admitted Sunday.

The spokeswoman for his Forza Italia party, Deborah Bergamini, initially said Berlusconi was expected to be hospitalized for a few days of tests after suffering an irregular heartbeat.

Berlusconi, who has had a pacemaker for years, called Zangrillo Sunday morning from Rome to report the ailment, and Zangrillo said he advised him to return to Milan and go directly to the hospital from the airport.

"He obeyed me in part. He wanted to exercise his right to vote and he only came to us in the afternoon," the doctor said.

Zangrillo said a team of doctors at the multi-disciplinary San Raffaele clinic ran a battery of tests and diagnosed a severe case of aortic insufficiency, which involves a leaking of the aortic valve of the heart, causing blood to flow in the reverse direction.

He will undergo the four-hour surgery by the middle of next week to substitute the valve, after which he will be in intensive care for one or two days before a period of rehabilitation that will last about a month. He said the mortality rate was around 2 percent.

Zangrillo said that after the rehabilitation, Berlusconi would be "better than before," from the point of view of vitality.

He said Berlusconi was being treated "like all other patients."

Berlusconi no longer holds public office due to a 2013 tax fraud conviction but remains the head of his Forza Italia party and has been active in mayoral campaigns under way.

Berlusconi's business interests include the Mediaset media empire, the Mondadori publishing house and the AC Milan Serie A soccer club that he has been trying to sell.