Combined with additional work pressures, it can increase blood pressure significantly, leading to an increased risk of long-term heart disease, says a US report.

A study of 72 regular coffee drinkers by researchers at the Duke University Medical Center found that they produced high levels of adrenalin and nonadrenalin hormones.

Professor James Lane, who took part in the research, told a behavioural medicine conference on Thursday: "Moderate caffeine consumption makes a person react like he or she is having a very stressful day.

"If you combine the effects of real stress with the artificial boost in stress hormones that comes from caffeine, then you have compounded the effects considerably."

Hormones

Over a two-week period, the coffee drinkers' adrenalin levels increased by 32% and a 14% rise in nonadrenalin levels.

Their blood pressure rose by an average three points.

The hormone levels remained high until nightime, even when coffee was last consumed before 1pm.

Professor Lane said a regular boost of three points in blood pressure could contribute to health problems.

Studies have shown that a rise of five points in diastolic blood pressure - the bottom number in blood pressure readings - can increase the risk of stroke by 34% and the chance of suffering a heart attack by 21%.

Professor Lane says most studies have looked at the effects of caffeine in the laboratory.

The new research, funded by the National Heart, Lung and Blood Institute, studied people in their normal working environment.

No significant risk

However, the British Heart Foundation (BHF) says moderate amounts of caffeine should not cause problems for healthy people.

It says other studies have shown there is no significant increased risk of coronary heart disease for people who drink up to six cups of coffee a day.

However, some people are more sensitive to caffeine and may experience changes in heart rhythm after drinking coffee.

And for people who suffer dizziness and tiredness because of low blood pressure, coffee can have beneficial effects if taken before a meal.