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Sadiq Khan was urged to tell motorists to leave their car at home today as filthy French air was set to worsen pollution in London.

City Hall put out “high” air pollution alerts for Westminster and the City today, as well as on some other busy roads, and “moderate” generally for the rest of the capital.

Green peer Baroness Jenny Jones tweeted: “Perhaps @MayorofLondon will call on drivers to leave their cars behind? Or just warn the rest of us to ‘breathe moderately’?!!”

Simon Birkett, founder of Clean Air in London, added: “MODERATE-HIGH #airpollution today as London stews in its own (diesel) juice. Don’t drive today!

“Relief on Tuesday as Atlantic air arrives.”

Experts at King’s College London stressed air flowing into London today is forecast to have travelled up through France and close to Paris.

“It’s likely we will import particulate pollution to add to local emissions poorly dispersed in near calm conditions,” they added.

“As a result, ‘Moderate” PM10 and PM2.5 particulate levels are expected, with ‘Moderate’ nitrogen dioxide also likely at busy roadside locations.”

London has been hit by a peak of toxic air in recent days, with a cloud of pollution, photographed from a helicopter on Friday, seen to be smothering the capital.

Advice from Public Health England for moderate air pollution is: “Adults and children with lung problems, and adults with heart problems, who experience symptoms, should consider reducing strenuous physical activity, particularly outdoors.”

Other people should “enjoy your usual outdoor activities”.

Only when dirty air levels become “high” should the general population consider reducing activity if experiencing discomfort.

The pollution drifting in from France is tiny particulates.

Scientists say PM2.5 particulate pollution is particularly dangerous as it can get deep into the lungs and seep into the bloodstream — causing heart and lung disease, cancer, and aggravating asthma.

Nitrogen dioxide pollution is produced locally, significantly from diesel vehicles.

When pollution gets bad in Paris, half of cars are sometimes banned from entering the city centre.