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People who smoke less than half a dozen each day are also 50% more likely to suffer heart failure, say American scientists.

The analysis of data from more than 20 long-term studies, involving over 300,000 participants, found that people who smoke just two to five cigarettes per day have a "substantially higher" risk of heart disease and death compared to people who never smoked - even years after they stub out the habit.

Previous studies have shown that smoking increases a person’s risk of developing cardiovascular disease.

But the researchers who conducted the new study said the exact relationship between how heavily a person smokes and their risks was still unclear - especially for low-intensity smokers.

Study leader Professor Michael Blaha said: "Today, more people are smoking fewer , but it’s still important to understand the cardiovascular risks and long-term benefits of quitting, even for individuals who aren’t smoking a pack a day."

Blaha’s team analyzed data from more than 300,000 adults enrolled in 22 studies which involved following participants for up to 20 years.

In that time, they documented more than 125,000 deaths and 54,000 cardiovascular events - such as heart attacks, strokes and heart failure.

The analysis showed that even very low-intensity smoking - defined as two to five per day - was associated with a 50% higher risk of heart failure and a 60% higher risk of death from any cause, compared to never smoking.

Blaha, of in Baltimore, said: "A person’s risk of cardiovascular events dropped most substantially in the first decade after quitting smoking and continued to decrease over time.

"However, even up to three decades later, former smokers may still exhibit higher risk compared to those who never smoked."

Considering that even occasional or very low-intensity smoking significantly increases a person’s risk of cardiovascular disease and death, the researchers concluded that quitting smoking at a younger age is the best way to decrease your risk, rather than reducing the .

They say their findings, published in the journal , reinforce established public health guidelines – that smokers should quit as early as possible instead of just cutting back – and emphasise the importance of smoking prevention schemes.

Blaha said: “This is one of the largest studies of cigarette smoking to date using the highest quality data in the cardiovascular epidemiology literature.

"It is remarkable how harmful smoking is – even low doses of smoking confer large cardiovascular risks."

He added: "As far as behaviour change, it is imperative to quit smoking as early in life as possible, as the amount of time passed since complete is more important than prolonged exposure to a lower quantity of cigarettes each day.�