A painkiller prescribed to millions of people can raise the risk of a heart attack or stroke by 50 per cent - even at low doses, warns new research.

Controversial pill diclofenac also increases rates of heart failure and irregular heartbeat or flutter among men and women of all ages, according to the study.

Diclofenac was recently withdrawn from being available over the counter in the UK because of concerns about serious side effects.

It is widely used to treat a range of conditions including headaches, back pain, arthritis and flu.

In January 2015 diclofenac was reclassified as prescription only. The Medicines and Healthcare products Regulatory Agency said the anti-inflammatory tablets carried a "small but increased" risk of heart problems.

But they can still be bought over the counter in most countries, including the US.

Now a groundbreaking study of more than six million people, the biggest of its kind, has linked them to "major cardiovascular events".

Cardiologist Dr Morten Schmidt, of Aarhus University Hospital, Denmark, said: "Diclofenac poses a cardiovascular health risk compared with non-use, paracetamol use, and use of other traditional non-steroidal anti-inflammatory drugs."

Writing in The BMJ, he says diclofenac should only be available with a prescription, like in the UK - and only them when also accompanied with an appropriate warning on the front of the pack.

His team found patients using diclofenac, commonly bought under the brand name Voltarol, were more likely to suffer a heart attack or stroke, or other cardiovascular condition.

Rates rose by 50 percent after 30 days among patients taking diclofenac. This was compared to those not taking any drug.

They also increased by 20 percent compared to those on ibuprofen or paracetamol, and 30 percent compared to those using naproxen.

Dr Schmidt said: "It is time to acknowledge the potential health risk of diclofenac and to reduce its use."

It belongs to a class of painkillers known as NSAIDs (non-steroidal anti-inflammatory drugs). About five million prescriptions for it are written annually in the UK, alone.

Its cardiovascular risks compared with those of other traditional NSAIDs have never been examined in large randomised controlled trials. That is because current concerns would make them unethical.

Dr Schmidt and colleagues took a different approach by analysing national registry data for more than 6.3 million middle aged adults in Denmark.

By tracking the individuals over two decades between 1996 and 2016 they showed

starting diclofenac was linked to an increased rate of irregular heart beat or flutter, stroke, heart failure and heart attack.

This was compared compared to taking paracetamol, or the NSAIDs ibuprofen or naproxen.

Starting diclofenac was also associated with an increased rate of cardiac death compared with those taking no NSAIDs. They were also more likely to suffer internal bleeding in the upper intestines.

Dr Schmidt said: "Treatment of pain and inflammation with NSAIDs may be worthwhile for some patients to improve quality of life despite potential side effects.

"Considering its cardiovascular and gastrointestinal risks, however, there is little justification to initiate diclofenac treatment before other traditional NSAIDs."

The researchers pointed out the absolute risk of heart attack, stroke or other cardiovascular disease still remained low for individual patients.

And as it was an observational study, no firm conclusions can be drawn about cause and effect.

But the study's sample size is larger than most previous analyses of observational and randomised studies taken together and provides strong evidence to guide clinical decision making.

The Medicines and Healthcare Products Regulatory Agency (MHRA) has said diclofenac should not be used by people with serious underlying heart conditions.

People who have suffered heart failure, heart disease or a stroke should stop using it completely.

Smokers and people with high blood pressure, raised cholesterol and diabetes have been advised to use the drug only after consulting their GP or pharmacist.

The MHRA sys diclofenac would continue to provide safe and effective pain relief, apart from patients in certain "at risk" groups.

In 2013, a major Oxford University study found that for every 1,000 people with a moderate risk of heart disease taking 150mg a day for a year, about three would experience an avoidable heart attack, of which one would be fatal.

A review by European health officials confirmed the finding, and said patients should no longer use diclofenac if they have a heart condition, or have previously suffered heart attacks or strokes.

Diclofenac also gives short-term relief from toothache and period pain. It blocks a substance in the body called cyclo-oxygenase, which produces chemicals in response to injury - causing pain, swelling and inflammation.