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A toxic air alert has been issued for London today as Mayor Sadiq Khan warned of “shocking and illegal” pollution levels on the capital’s streets.

City Hall issued the “high alert” caution on Wednesday morning, with high pollution levels forecast for the rest of the day.

It blamed the toxic air on pollution blowing towards London from "industrialised" areas of Europe including parts of France and Germany.

The alert is believed to be the 7th warning issued since they were introduced over a year ago, with pollution levels having reached critical levels in London.

It comes just weeks after Mr Khan proposed a series of new measures aimed at tackling the capital’s worsening pollution crisis.

London Pollution - In pictures 15 show all London Pollution - In pictures 1/15 PA 2/15 Sunrise: Pollution lingered over London on Tuesday morning Jeremy Selwyn 3/15 A cyclist wearing a face mask rides between taxis taking part in a protest to highlight congestion and air pollution Getty Images 4/15 A smog filled traffic rush hour in the East End of London Jeremy Selwyn 5/15 Pollution detector App 'Blue Air' near the Houses of Parliament Blue air 6/15 Thick smog hangs over London's Docklands PA 7/15 Traffic pollution in North London Jeremy Selwyn 8/15 Air pollution surrounds The Shard in London Jeremy Selwyn 9/15 Getty Images 10/15 Sunrise over a misty polluted London from Primrose Hill Jeremy Selwyn 11/15 St. Paul's Cathedral is seen among the skyline through the smog AFP via Getty Images 12/15 A thick layer of smog was visible above London William Smith/@williamsmithorg 13/15 A cold layer of air can be seen trapping pollution close to the ground and across the London skyline Pete Buckney 14/15 This image taken near the Royal Observatory in Greenwich shows a thick layer of fog hanging over London Anna Rolls 15/15 The City of London covered in smog seen from Hampstead Heath Getty Images 1/15 PA 2/15 Sunrise: Pollution lingered over London on Tuesday morning Jeremy Selwyn 3/15 A cyclist wearing a face mask rides between taxis taking part in a protest to highlight congestion and air pollution Getty Images 4/15 A smog filled traffic rush hour in the East End of London Jeremy Selwyn 5/15 Pollution detector App 'Blue Air' near the Houses of Parliament Blue air 6/15 Thick smog hangs over London's Docklands PA 7/15 Traffic pollution in North London Jeremy Selwyn 8/15 Air pollution surrounds The Shard in London Jeremy Selwyn 9/15 Getty Images 10/15 Sunrise over a misty polluted London from Primrose Hill Jeremy Selwyn 11/15 St. Paul's Cathedral is seen among the skyline through the smog AFP via Getty Images 12/15 A thick layer of smog was visible above London William Smith/@williamsmithorg 13/15 A cold layer of air can be seen trapping pollution close to the ground and across the London skyline Pete Buckney 14/15 This image taken near the Royal Observatory in Greenwich shows a thick layer of fog hanging over London Anna Rolls 15/15 The City of London covered in smog seen from Hampstead Heath Getty Images

He announced plans to audit the capital’s most polluted schools, with thousands of children being exposed to illegal levels of pollution.

In October, a £10 ‘T-charge’ will also be introduced for drivers using the most polluting cars in central London.

Announcing the introduction of Wednesday’s alert on Twitter, Mr Khan posted: “London: the shocking and illegal state of our toxic air means another high air pollution alert.”

London breached its legal limits for toxic air set by the European Union for the whole of the year just five days into 2017.

Earlier this year, the London Mayor took the unprecedented step of issuing the top “black” alert in London as pollution soared in parts of the capital.

City Hall has promised “hard-hitting” measures to combat the capital’s toxic air, with £250,000 in funding having already been set aside for audits of London most polluted schools.

Larger projects such as improved road layouts and pedestrianisation could also be brought in by councils.

It is hoped the introduction of such schemes could help thousands of children across London.

King’s College London air quality researchers said Wednesday’s toxic air is being brought in by a large high pressure system centred over northern Scandinavia.