They warn that dishing out the anti-impotence drug Viagra to such men - who may exhibit no obvious symptoms of heart disease - could pose a serious risk to their health.

In some instances impotence, or the inability to achieve or maintain an erection, is linked to disease of the blood vessels in the penis.

Researchers from the Minneapolis Heart Institute Foundation believe that patients who suffer from damaged blood vessels in the penis are at greater risk of having similar damage elsewhere in the body, including the heart, brain, legs and kidneys.

They are recommending that doctors carry out tests on all impotent men to determine whether they have damaged blood vessels in their penis, and whether they need cardiac assessment as well.

Researcher Dr Marc Pritzker said: "Because the blood vessels that supply the penis are narrower than arteries in other areas of the body, atherosclerosis - the disease process that leads to heart attacks and strokes - may manifest itself as erectile dysfunction before the disease becomes apparent in other arteries.

"This provides a wonderful opportunity for strong preventive programs that could reduce the risk of heart attacks and strokes."

Dr Pritzker and his team reviewed the histories and test results of 50 men with erectile dysfunction who had sought prescriptions for the anti-impotence treatment Viagra, and who were referred on by their doctors for specialist evaluation.

Although none of the men had symptoms of heart disease, 20 of them were found to have significant blockages in heart arterties, which are associated with chest pain and an increased risk of heart attack.

50% of cases may mask heart problems

Dr Pritzker said it would be wrong to create the impression that every case of impotence was linked to heart disease.

But he estimated that up to 50% of cases are the result of blood vessel disease.

He said: "A man having regular sexual activity who experiences a consistent change in erectile function may be demonstrating signs of atherosclerosis where arteries become clogged and the heart muscle does not receive enough blood.

"As we become more thorough in our questioning of patients, it is not uncommon to hear that erectile dysfunction preceded the onset of heart disease by a year or more."

Dr Pritzker warned that there could be consequences for men who blithely treat their impotence with Viagra without examining the state of their overall health.

He said: "Sexual activity is a form of exercise that can raise your heartbeat and your blood pressure.

"If you have erectile dysfunction, it shouldn't be treated with a prescription alone."

The British Heart Foundation issued a statement which said: "These findings are to be welcomed as strengthening the case for careful screening of patients with impotence, but no apparent previous risk factors for cardiac disease, to find out whether or not heart disease may be present or developing.

"All patients with impotence should have a medical assessment to exclude any underlying physical problem."

Impotence affects more than five million men in the UK.