Shoppers thronging London's Oxford Street are exposed to the highest levels of toxic pollutant nitrogen dioxide (NO₂) in the world, claim scientists.

A monitor installed in the central London street found high levels of the invisible gas, which is produced by diesel engines.

The station has recorded NO₂ levels of 135 micrograms per cubic metre of air (mcg/mᶟ) so far this year, more than four times the European Union's limit of 40 mcg/mᶟ.

The figure is an average, which includes nighttime periods when there is less traffic and thus less pollution.

During daytime hours when the street is busy with shoppers and workers pollution levels are much higher, with 436 mcg/mᶟ in recorded in March.

The high levels of NO₂ are due to Oxford Street being constantly jammed with lorries and taxis said the scientists, whose diesel engines generate large amounts of the gas as they haltingly proceed.

Asthma sufferers who were asked to spend time in Oxford Street as part of a medical study experienced lung inflammation, sharp decline in lung function, shortness of breath, and other symptoms of an impending attack.

High pollution has also been linked by scientists to death from heart attacks and strokes.

The levels of NO₂ in London were higher than those recorded in Chinese and Indian cities. However the UK capital had lower levels of particulates: which are harmful particles from unburnt fuel that cause smog.

In modern diesel engines NO₂ is used to burn off particulates, accounting for the high levels of the gas in London.

David Carslaw, who researches vehicles emissions and air quality at King's College London, told the Sunday Times: "To my knowledge this level is the highest in the world in terms of both hourly and annual mean. NO₂ concentrations in Oxford street are as high as they have ever been in the long history of air pollution."