Air pollution in cities such as Delhi and Beijing is killing millions of people and threatened to overwhelm public services in countries across the globe that will have major financial implications for governments, the Geneva-based organisation said.

New WHO figures scheduled for release in February are expected to show that air pollution has worsened since 2014 in hundreds of already blighted urban areas. The data is taken from 2,000 cities.

"We have a public health emergency in many countries from pollution. It's dramatic, one of the biggest problems we are facing globally, with horrible future costs to society," said Maria Neira, head of public health at the WHO, told The Observer.

She added: "Air pollution leads to chronic diseases, which require hospital space. Before, we knew that pollution was responsible for diseases like pneumonia and asthma. Now, we know that it leads to bloodstream, heart and cardiovascular diseases, too - even dementia".

"We are storing up problems. These are chronic diseases that require hospital beds. The cost will be enormous."

According to Nicholas Stern, the IG Patel Professor of Economics and Government at the London School of Economics, air pollution is an important factor in climate change: "Air pollution is of fundamental importance. We are only just learning about the scale of the toxicity of coal and diesel. We know that in China, 4,000 people a day die of air pollution. In India it is far worse. This is a deep, deep problem".

Amid continuing concerns over pollution around the Heathrow airport in London, the David Cameron government recently announced plans to discourage heavy polluting vehicles such as old buses, coaches and lorries from entering Clean Air Zones in five cities by 2020.

Several British cities -- including London -- face pollution, prompting promises in election manifestos and rulings by the Supreme Court. Diesel is increasingly seen as a dirty fuel responsible for most of the pollution.