The number of people with hypertension has almost doubled in 40 years to over 1.1 billion worldwide, scientists have said, with the burden of the condition shifting from the rich to the poor.

In the largest study of its kind analysing blood pressure in every nation between 1975 and 2015, the scientists said that it has dropped sharply in wealthy countries - possibly due to healthier diets and lifestyles - but risen in poorer ones.

The increases are especially significant in Africa and South Asia, the researchers said, and could be partly due to poor nutrition in childhood.

Led by World Health Organisation researchers working with hundreds of scientists internationally, this study covered blood pressure measurements from nearly 20 million people and was published in The Lancet.

In Europe, Britain had the lowest proportion of people with hypertension in 2015. South Korea, the United States and Canada had the lowest hypertension rates in the world.

More than half the world's adults with hypertension in 2015 lived in Asia, the study estimated. Some 226 million people in China have hypertension, it said, as do 200 million in India.

"High blood pressure is no longer related to affluence - as it was in 1975 - but is now a major health issue linked with poverty," said Majid Ezzati, a professor at Imperial College London's school of public health.

He said that while he could not be sure of why the data showed hypertension as more of a problem in poorer countries, it may be partly due to overall better health and more consumption of fruit and vegetables in wealthy societies.

In rich countries, the condition is also caught more frequently and earlier, and managed more effectively with medicines, Ezzati said.

Hypertension is caused by a number of factors including having a diet high in salt and low in fruit and vegetables, and not getting enough exercise.