A high pollution warning has been issued for London for the first time since August 2020.
The mayor of London warned of “extremely dangerous” toxic air amid “imported pollution from the continent” alongside a build up of local emissions.
Sadiq Khan said the new alert would be in place until Thursday.
People with heart and lung problems are being advised to avoid physical exertion during the alert period, while Londoners are urged to avoid unnecessary car journeys or burning wood and garden waste.
Mr Khan said: “Toxic air is extremely dangerous for Londoners – and I’m doing everything in my power to tackle it.
“Today we have seen the capital experiencing high levels of air pollution and are expecting this to continue for two days.
“For this reason, I am issuing a ‘high’ air pollution alert from today until Thursday.
“I’m urging Londoners to look after each other by choosing to walk, cycle or take public transport, avoiding unnecessary car journeys, stopping engine idling and not burning wood or garden waste, all of which contributes to high levels of pollution.
“This is particularly important in order to protect those who are more vulnerable to high pollution.
“While this alert is in place people with heart and lung problems should avoid physical exertion.”
Mr Khan earlier this month announced plans to extend the city's Ultra-Low Emission Zone (ULEZ) to the whole of Greater London.
The mayor said the policy would help tackle the "triple challenge" of air pollution, the climate emergency, and congestion.
Under the plans, from next year motorists in vehicles that do not meet emissions standards would have to pay £12.50 a day to drive in Greater London.
It came after new research suggested toxic air in the capital caused more than 1,700 hospitalisations for asthma or serious lung conditions in just three years.
Over a third of these were children admitted for asthma, scientists from Imperial College London found.
The UK government describes poor air quality as the “largest environmental risk to public health” in the country.
Exposure can cause a wide range of diseases, including respiratory illnesses and cancer, and exacerbate existing conditions such as asthma.
