The study also found that response to painkilling drugs is also dependent on genes, with a wide variation between different people.

The end product could be drugs which are tailored to meet each individual's pain response and painkiller tolerance, reducing the risk of overdose, and increasing the amount of relief.

The science could even help predict how likely any one person was to become hooked on opiate drugs, whether given as medicine or taken for recreation.

About six million people in the UK are thought to suffer from some sort of constant pain.

Measure your own

At the moment, assessment of pain is a subjective process - patients are asked to score their own pain.

Pain response is governed by genes, say scientists

Dr George Uhl, who experimented on both mice and humans, said: "It's rare to find a gene where the animal evidence for its effect is so strong or has such a clear carry-over to human studies."

The key ingredient is a "mu opiate receptor gene", which bonds with the body's natural opiate painkillers.

It also bonds with opiates like morphine which are given to relieve pain.

Dr Uhl found that the more receptors that mice had, the less sensitive they were to a mildly painful stimulus.

Mice who had all their mu receptors knocked out were highly sensitive.

Scepticism about genetic link

Scans at labs in Johns Hopkins University in the US found that the number of mu receptors in humans differ dramatically - some have almost twice the number in certain brain areas.

Dr Uhl said: "People have long been sceptical that pain has a genetic basis. They don't notice that sensitivity can vary because the differences can be subtle, and masked by a strong emotional response to pain.

"Many assume the way people respond is voluntary. 'Just put up with it' has been a common recommendation."

Dr Beverley Collett, Honorary Secretary of the Pain Society and a consultant in pain management, said the research could prove highly significant.

She said: "Many people don't get the opiate pain relief they need because physicians are scared they will become dependent on the drug.

"If a method could be developed of telling how likely this was, it would be very useful."

However, she warned that any genetic test for pain sensitivity could be very expensive, and beyond the budgets of hospitals who do not give pain management the priority it deserves.