Pollution levels have reached level 9 (high) this morning in the South East, Greater London and Eastern England, the Department for Environment, Food and Rural Affairs (Defra) has reported on its website.
Defra ranks air pollution from one to 10, with one being the lowest and 10 the highest.
Very high levels of pollution are also forecast later today for the East Midlands.
Those with lung and heart conditions have been told to avoid strenuous activity outdoors while people suffering symptoms of pollution - including sore eyes, coughs and sore throats - should cut down the amount they do outside, health experts said.
Asthmatics have been warned of the need to use their blue reliever inhalers more often as they could be prone to attacks over the next few days.
Around two-thirds of the 3.6 million people with asthma find that air pollution makes their asthma worse.
The advice, from Public Health England (PHE), Asthma UK and Defra, comes after a warning that parts of England are experiencing the highest level of air pollution ever recorded by Defra.
Across much of England, moderate to high air pollution levels were measured yesterday, with level 8 (high) in the South East and Eastern regions and level 7 (high) in Greater London.
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