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Many more people will die of heart problems as global warming continues, experts are warning.

Climate extremes of hot and cold will become more common and this will puts strain on people's hearts, doctors say.

A study in the British Medical Journal found that each 1C temperature drop on a single day in the UK is linked to 200 extra heartattacks.

Heatwaves, meanwhile, increase heart deaths from other causes, as shown by the events in Paris during summer 2003.

Over 11,000 people died in France's heatwave in the first half of August of that year when temperatures rose to over 40C.

Many of these were sudden cardiac deaths related to heart conditions other than heart attack.

That same summer, record-breaking temperatures led to 2,000 excess deaths in the UK.

And experts predict that by the 2080s events similar to these will happen every year.

The risks posed by extreme spells of hot and cold are largely within two weeks of exposure and are greatest for the most frail - theelderly and those with heart problems already, say experts.

In the BMJ study, researchers from the London School of Hygiene & Tropical Medicineanalysed data on over 84,000 patients admitted to hospital with a heartattack between 2003 and 2006 and compared this with daily temperaturesin England and Wales.

They found that a 1C reduction in average daily temperature was linked with a cumulative 2% increase in risk of heart attack for 28days, even in the summer.

Although a 2% increase in risk may sound small for any given individual, for the population of the UK it equates to 200 extra heartattacks a day, say the researchers.

Most of the casualties were people in their 70s and 80s, but people who had been taking aspirin long-term appeared to be lessvulnerable for some reason.

The researchers speculate that the cold may make blood more prone to clotting, and that this raises heart attack risk.

It would also explain why the blood-thinning drug aspirin might offer some protection.

Lead researcher Krishnan Bhaskaran and his team say further studies need to be conducted to see what measures could be used to avoidthe increased risk, such as advising patients, particularly theelderly, to wear suitable clothing and to heat their homes sufficiently.

Last year's low temperatures saw the highest number of "excess deaths" - the number of those who perished over and above whatis normal for the time of year - for nearly a decade.

The 36,000 "excess deaths" in England and Wales during the winter of 2008/09 represented a rise of nearly 50% from the previousyear.

In an accompanying editorial in the BMJ, Dr Paola Michelozzi and Manuela De Sario, of the LazioRegion Department of Epidemiology in Rome, say although rising globaltemperatures will bring some health benefits, such as lower cold-relatedmortality, any benefits will be outweighed by the health risks linkedto heatwaves.

"Actions to reduce greenhouse gases based on lifestyle changes at the population and individual level may have substantialbenefits for health and climate protection.

"For example, lowering saturated fat intake by reducing consumption of animal products is a healthy food choice recommended inprevention guidelines for coronary heart disease and a recognisedstrategy to reduce greenhouse gas emission," they say.

Ellen Mason, of the British Heart Foundation, said: "Although the increased risk is small, if there is a nationwide drop in averagetemperature it could equate to a significant number of heart attackseach day.