A red alert has been issued in eight London boroughs, including Kingston and Wandsworth, with millions of residents warned to stay indoors as toxic air sweeps in from Germany.
The warning to stay indoors was issued by London Mayor Sadiq Khan, who has told residents to avoid busy roads in the capital.
Kingston, Wandsworth and Hounslow are among the boroughs worst affected, with moderate warning sent out for the rest of the capital.
The cloud of pollution is believed to be heading from Germany’s industrial region, with children and the elderly considered most at risk.
Mr Khan said: "The extent of our air quality crisis means that I have today triggered a London-wide air quality alert, ensuring Londoners have all the information possible.
"It’s crucial that Londoners who are vulnerable, such as asthma sufferers, are able to take appropriate measures to protect themselves."
Air quality warnings have reportedly been sent to about 2,500 bus countdown signs across London today.
London breached its legal limits for toxic air for the entire year in the first five days of 2017, with Putney High Street one of the first places to breach the limit.
City Hall issues a warning if more than four boroughs, excluding the City of London, are forecast to hit high pollution levels.
The air toxicity must also hit a score of between seven and nine out of 10.
Westminster, Kensington and Chelsea, Hammersmith and Fulham, Tower Hamlets, and the City of London have also had alerts issued.