One in 10 heart attack sufferers ignore the symptoms for two days or more before seeking medical help, a charity has found.
The British Heart Foundation (BHF) said people are putting their lives at risk by delaying seeking urgent assistance.
A poll of heart attack survivors found that one in three initially thought they were suffering from indigestion.
And 50% did not seek medical help for more than an hour.
This includes 9% who delayed for two to three hours, 7% who delayed between three and five hours and 11% who waited two days or longer before seeking medical assistance.
The BHF is urging people to be more aware of the signs of a heart attack - which occurs when a blood clot forms in a narrowed coronary artery, cutting off the blood supply to the heart.
Symptoms include chest pain, pain in other parts of the body such as the arms, feeling lightheaded or dizzy, sweating, shortness of breath, nausea or vomiting, coughing or wheezing and an overwhelming sense of anxiety.
The charity said that nearly half of potentially salvageable heart muscle is lost within one hour of the coronary artery being blocked.
But only 26% of the 500 heart attack survivors surveyed managed to get treatment in that time.
"It's extremely alarming that the majority of people who suffer heart attacks mistake their symptoms for something less serious and delay getting medical help," said BHF chief executive Simon Gillespie.
"Every second counts when someone has a heart attack. The sooner people recognise their symptoms and call 999, the better their chance of recovery."
The charity also said that more research is needed to improve ways of preventing, diagnosing and treating heart attacks.
Mr Gillespie added: "Research advances mean seven out of ten people now survive a heart attack. But most heart attacks occur without warning and we have no way of predicting when they will strike.
"We need to accelerate research into improving our understanding of the furring of the arteries that causes heart attacks and develop better ways of preventing them. Also, minor heart attacks which are often a prelude to a much more serious one, can be difficult to diagnose.
"We therefore need more effective ways of diagnosing them so people at risk get the life-saving treatment they need."