Pictures of London carpeted in fog as mayor issues unprecedented air pollution alert
The air quality index stands at 10 out of 10, which means pollution is higher than ever under Khan's new measurement system, according to an announcement from the mayor's office.
The pollution is being caused by cold and settled weather. City Hall said low wind speeds are leading to the "poor dispersal of local pollutants" meaning that they have built up over the course of several days.
"Today the shameful state of London's toxic air has meant that I am forced to trigger the first 'very high' air pollution alert under my new comprehensive alert system," Khan said.
"This is the highest level of alert and everyone - from the most vulnerable to the physically fit - may need to take precautions to protect themselves from the filthy air."
Freezing fog covered most of southern England on Monday, contributing to the high pollution levels in London. Scroll on to see dramatic images of the fog, which has shrouded London landmarks and grounded flights from airports.