The government has warned people in London to avoid outdoor exercise on Friday due to “very high” pollution.
High pressure across western Europe means that pollution will not be blown away and will hit the highest level on the scale.
The worst of the pollution will be in the centre of the capital from Stratford to Hammersmith.
Official government advice had stated that healthy people should “reduce physical exertion, particularly outdoors, especially if you experience symptoms such as a cough or sore throat.”
Earlier this week Sadiq Khan warned of the high levels of car use in London. He said: “If we do not double down on our efforts to deliver a greener, more sustainable future, we will replace one public health crisis with another – caused by filthy air and gridlocked roads.”
The high pollution on Friday is expected to return to normal levels by the weekend.