Alarming levels of pollution on London's streets have been highlighted in a major study.

Scientific tests have revealed how much of the airborne irritant oxides of nitrogen (NOx) is created by traffic.

The toxin, which can cause breathing disorders, asthma and strokes, is also found in cigarette smoke. Experts are alarmed that NOx often soars way beyond safe limits during peak periods - often on smoggy, hot days.

In Marylebone Road on 28 July, one of the hottest days of the year, NOx levels rose to 1,912 microgrammes per cubic metre, the equivalent of motorists and pedestrians breathing in four cigarettes a minute.

Researchers from King's College, University Of London, found a normal day's exposure can be the equivalent of smoking 15 cigarettes. Pollution hotspots include Marylebone Road, where daily vehicle emissions are so concentrated that pedestrians and those with offices or homes on the roadside are exposed to the NOx equivalent of more than 30 cigarettes. Other affected areas include King's Road (29 cigarettes a day), and Hammersmith Broadway (27.3 cigarettes).

Scientist Eric Johnson, who worked on the study, said: "These figures give a clear indication of pollution in London. Those living or working next to one of the hotspots are constantly exposed to high levels but, even if you come in and out of the capital, the pollution adds up. We should be very concerned by this."

Andrew Ford, of the Liquefied Petroleum Gas Association which analysed the King's College data, said: "This pollutant is strongly linked to breathing problems and 40,000 premature UK deaths a year are attributed to poor air."

The index shows that at 57 out of 85 London sites, annual average levels are above the Government's recommended limit of 40.11 microgrammes of NOx, the equivalent of smoking 12 "light" cigarettes in 24 hours.