"He will be having surgery early in the week," his wife, Senator Hillary Clinton said last night after visiting her husband in hospital.
Mr Clinton, 58, went to his doctor on Thursday and was told to have an angiogram, which was administered yesterday and revealed a "significant blockage", said a statement from his office, although the spokesman did not think that Mr Clinton had had a heart attack.
The television news networks ABC and CBS reported that Mr Clinton was told he needed a quadruple heart bypass.
His office said the former president "went to Northern Westchester hospital yesterday afternoon after experiencing mild chest pain and shortness of breath. Initial testing was normal and he spent the night at home in nearby Chappaqua, New York.
"After undergoing additional testing this morning at Westchester medical centre, doctors advised he should undergo bypass surgery."
Tony Blair, who suffered heart problems himself last year, sent a personal message to Mr Clinton on hearing the news.
Mr Clinton had cancelled a two-day trip with Mrs Clinton, across upstate New York and was taken to a Manhattan hospital.
The severity of Mr Clinton's coronary problem was discovered at Westchester medical centre, where he was found to have multiple lesions.
The former first lady made a brief appearance yesterday at the New York state fair in Syracuse, after which she noted that her husband was hospitalised and would need heart surgery. She apologised for leaving right away. "He's in excellent hands," she said.
Mr Clinton, the 42nd US president, promised at the Democratic national convention in July to be a "foot soldier" in John Kerry's race against the Republican president, George Bush. The former president received a rousing welcome from delegates after being introduced by his wife.
"He's going to be fine," Mr Kerry told a campaign rally in Newark, Ohio, yesterday.
"But every single one of us wants to extend to him our best wishes, our prayers and our thoughts, and I want you all to let a cheer out and a clap that he can hear all the way to New York," the Democratic contender added to cheers.
In early 2001, shortly after Mr Clinton's presidency ended, he had a cancerous growth removed from his back which turned out to be basal cell carcinoma, the most treatable form of skin cancer.
In 1996, he had had a pre-cancerous lesion removed from his nose, and a year before that had a benign cyst taken off his chest.
Other than that, Mr Clinton has had the normal health problems that often accompany ageing - periods of slightly elevated cholesterol and hearing loss - and an appetite for junk food. In 1997, he was fitted with hearing aids. He has also suffered from allergies.
He has been active in recent years, giving speeches and completing and promoting his best-selling memoir.
His charisma and political clout were seen as a substantial potential asset to Mr Kerry in the coming hectic weeks of the presidential election campaign.
Mrs Clinton is gearing up for a 2006 reelection bid seen by many as a prelude to running for president in 2008 should Mr Kerry fail in his run for the White House this year.
In LittleRock, Arkansas, Mr Clinton's mother-in-law, Dorothy Rodham, said he had called her after the chest pains started and sounded good.
"He sounded wonderful as usual and very upbeat as he always is," she said. "I just told him how much I love him." She said she did not know if he had been in the hospital when he called.
Mr Clinton called his stepfather, Dick Kelley, at his Hot Springs, Arkansas, home.
"He's very gung-ho and optimistic about what's going to happen," said Mr Kelley.
"I don't know if he was at the hospital. He just wanted me to know before the press, but the press knew before I did," he added.