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Mayor Sadiq Khan today put London on toxic air alert for at least three days.

City Hall sent out warnings that air pollution could rise today to “moderate” in 17 boroughs and the City.

It is then set to hit a “high” peak in Westminster and the Square Mile tomorrow before reducing to “moderate” in these two areas on Friday.

But some of the capital’s busiest main roads will be blighted by “high” pollution on all three days, according to the airtext forecasts put out by the Mayor.

Mr Khan said: “London’s dirty air is a public health emergency.

“We will continue to use all the technology at our disposal to inform Londoners about levels of air pollution in their neighbourhoods.”

The Department for Environment suggested the pollution episode could last until at least Sunday.

Official health advice on days of “moderate” air pollution says adults and children with lung problems, and adults with heart problems, who experience symptoms, should consider reducing strenuous physical activity.

If it reaches a “high” level, these groups of people should reduce strenuous physical exertion, particularly outdoors, and particularly if they experience symptoms.

People with asthma may find they need to use their reliever inhaler more often. Older people should also reduce physical exertion.

For the general public, the “high” pollution advice is that anyone experiencing discomfort such as sore eyes, a cough or sore throat should consider reducing activity, particularly outdoors.