London mayor Sadiq Khan has issued a "very high" air pollution alert - the first since he took office in May.
It's also the highest level of alert possible under Khan's new warning system, and is one step above the "high" alert the mayor issued last week.
Earlier this month, the London Air Quality Network said the capital had broken the air pollution limits set for the whole of 2017 just five days into the year.
Last year Khan set out plans to crack down on London's most polluting vehicles, with an emissions surcharge for older polluting vehicles to be introduced in October.
According to the mayor's alert, pollution levels are currently more than double the legal hourly limit, caused by the cold, calm and settled weather - apparently, "low wind speeds are leading to poor dispersal of local pollutants meaning pollution has built throughout the course of several days".
Pollution levels have also been heightened by "an unusually high amount of domestic wood burning" on Sunday afternoon and evening.
The worst affected boroughs are Camden, Westminster and the City of London.
“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," he 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.
“Safeguarding the health of Londoners from pollution is my number one priority – and we are implementing the boldest policies of any city in the world to get to grips with the awful problems we inherited."
He added: “We’re delivering the strongest emission measures to clean-up our bus fleets, charging for the dirtiest most toxic diesels, and bringing forward and then extending the Ultra Low Emission Zone.
“The government urgently need to do their bit. They need to devolve more powers to London and introduce a national diesel scrappage scheme to rid our streets of the dirtiest vehicles. They also need to reform vehicle excise duty and bring in a new Clean Air Act that finally tackles this problem and means that Londoners don’t have to be afraid of the air we breathe.”