They have isolated a naturally-occuring substance in cocoa that they say stops the “root cause” of the irritating condition.

Some 7.5 million people in Britain suffer from a persistent cough every year according to a recent review, defined as one that lasts more than two weeks after the underlying cause disappears.

At the moment most medications to control the symptoms are opiate-based ones like cough syrups containing codeine, a narcotic.

But in October the Medicines and Health products Regulatory Agency (MHRA) said under-18s should not take such medication, because the drawbacks outweighed the benefits.

Now a British-based drugs company called SEEK is helping develop a medication based on a drug called theobromine, that it says “has been shown to inhibit the inappropriate firing of the vagus nerve, which is a key feature of persistent cough”.

It is found in “significant quantities in cocoa-based products”, said the firm

Professor Alyn Morice from the Hull Cough Clinic, said: “Due to the drawbacks of current opioid drugs such as codeine, we are in desperate need of a non-opioid treatment with a drastically improved side effect profile for patients.”

The drug is already sold in South Korea, and SEEK hopes it will be on sale in British pharmacies within two years.

Prof Morice said that while it was “theoretically possible” to get enough theobromine in a bar of dark chocolate to alleviate a cough, studies had yet to be done to reveal the exact dose required.