People at a high risk of having a heart attack do not know when to dial 999, heart specialists have warned.
Cardiac arrest can occur within 10 minutes of symptoms
They say patients with heart problems are waiting too long before seeking help because they cannot distinguish between angina pain and a heart attack.
Experts at Bristol Royal Infirmary recommend delaying only five minutes after taking a nitrate spray to relieve symptoms before calling an ambulance.
Writing in the BMJ, they admit current medical advice is "confused".
A recent campaign by the British Heart Foundation urged people to call 999 immediately if they experienced chest pain, after research showed 40% of people would wait before calling for an ambulance.
But among people with heart disease, who commonly suffer from chest pains, messages have been more confused.
The BHF advises taking three doses of a glyceryl trinitrate (GTN) spray at five-minute intervals before calling for help - a wait of 15 minutes.
Manufacturers of GTN sprays also give varying advice regarding how long people should wait before seeking help.
The guidance takes into account the fact that overcaution in these patients would overwhelm emergency departments.
However Dr Paul Walker, consultant cardiologist at Bristol Royal Infirmary, said patients suffering a heart attack needed quick attention and should not wait as long as 15 minutes.
ADVICE FOR HIGH-RISK PATIENTS
Take two doses of your GTN spray
Wait five minutes
If pain has not resolved, call an ambulance immediately. Do not try to drive or contact a friend or relative first
He pointed to recent research which showed the average time from onset of symptoms to cardiac arrest is just 10 minutes.
Dr Walker also believes that GTN sprays should be prescribed more widely in patients with diabetes, high blood pressure and high cholesterol.
"People at high risk should take two sprays and wait five minutes and if their pain hasn't gone they should call an ambulance - they shouldn't call a friend or try to drive themselves.
"This should be standard advice."
He said GTN sprays were currently given routinely only to those who have already had a heart attack.
"We advise our GPs to prescribe the spray to high risk patients, it costs next to nothing, they can carry it in their pocket and it lasts up to two years."
Call 999 immediately if you experience any of the following symptoms:
Central chest pain
Pain in your arms, neck or jaw
Feeling sick or sweaty
Feeling short of breath
"There are four million people at high risk and 70% of deaths occur in those at high risk," he added.
Dr Mike Knapton, director of prevention and care at the British Heart Foundation said: "Calling 999 immediately when a person suspects they are having a heart attack is sound advice whether the individual has been previously diagnosed with heart disease or not.
"However, we acknowledge that advice can be confusing for people who know they have heart disease.
"Heart patients are prone to getting chest pain frequently - angina - and need clear guidance on how to distinguish angina pain from the symptoms of a heart attack."
"This is crucial as we know that heart disease patients tend to delay a long time before calling for an ambulance when they have a heart attack."
He said the BHF's specific advice to heart patients was given as a guide only and was intended to be used alongside one to one advice from a doctor about recognising patients' particular symptoms.
Dr Terry McCormack said the problem was that many people did not call the ambulance for hours or even days and altered advice would probably not change that.
He added that the idea of handing out GNT sprays more widely was "interesting" but he would like to see more evidence that it would be effective and beneficial.
A separate study published in the BMJ suggests up to 1.5 million people may have wrongly been told they are at high risk of heart disease.
Researchers found current "risk calculators" used by GPs to determine whether an individual would benefit from anti-cholesterol drugs statins are flawed because they are based on US data from 20 years ago.
Analysis of more than a million adults aged between 34 and 75 in the UK found that white middle-aged men have a lower risk of heart disease than previously thought but the risk in women from deprived areas is underestimated.