The 93-year-old has been rushed to a hospital outside Tel Aviv. He has been sedated and is breathing with the help of a respirator, his office said. He is now expected to undergo a CT scan.
Shimon Peres had been described as conscious and stable when he arrived at the Sheba Medical Center in Tel Hashomer.
In January, the former president underwent successful minor surgery at the same hospital after suffering a small heart attack.
Israeli PM Benjamin Netanyahu tweeted: “Shimon, we love you and the entire nation is wishing for your recovery.”
Shimon Peres has held almost every major political office since Israel was founded in 1948, and was the architect of Israel’s secret nuclear program.
He twice served as prime minister and was president from 2007 to 2014.
Shimon Peres won the Nobel Peace Prize in 1994 for his role negotiating the Oslo peace accords with the Palestinians a year earlier, a prize he shared with PM Yitzhak Rabin, who was later assassinated, and Palestinian leader Yasser Arafat.
Despite his age, Shimon Peres has maintained an active public schedule, mostly through his non-governmental Peres Centre for Peace, which promotes closer ties between Israel and the Palestinians.
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