Watermelons are loaded with an organic compound called citrulline that can trigger blood vessels to relax, much like Viagra does, according to scientists from Texas A&M University in the United States.

Dr Bhimu Patel, who led the study, said that when watermelon is consumed, citrulline is converted to an amino acid called arginine which improves circulation.

Dr Patel, who is director of the university's Fruit and Vegetable Improvement Centre, said: "The more we study watermelons, the more we realize just how amazing a fruit it is in providing natural enhancers to the human body.

"We've always known that watermelon is good for you, but the list of its very important health benefits grows longer with each study.

"Arginine boosts nitric oxide, which relaxes blood vessels, the same basic effect that Viagra has, to treat erectile dysfunction and maybe even prevent it.

"Watermelon may not be as organ specific as Viagra but it's a great way to relax blood vessels without any drug side-effects."

He added that eating watermelon could also improve heart health, the immune system and even help people who suffer from obesity and type 2 diabetes.

Beneficial ingredients in watermelon and other fruits and vegetables are known as phyto-nutrients, naturally occurring compounds that are able to react with the human body to trigger healthy reactions.

While there are many psychological and physiological problems that can cause impotence, extra nitric oxide could help those who need increased blood flow, researchers found.

Citrulline is found in higher concentrations in the rind of watermelons than the flesh.

Because the rind is not commonly eaten, Dr Patil's team are working to breed new varieties with higher concentrations in the flesh.

Almost 92 percent of watermelon is water, but the remaining 8 percent is loaded with lycopene, an anti-oxidant that protects the human heart, prostate and skin health.