London has set a modern record for pollution after cold windless weather trapped emissions over the city.

For several moments between 17 to 24 January air quality was worse in the UK capital than in Beijing, which is infamous for smog.

Particulate matter was concentrated at 197 micrograms per cubic metre in London and 190 in Beijing.

An air quality index maintained at King's College London, which started in 2012, found it was at its worst in history at 20 different sites.

Pollution levels were twice the legal hourly limit, leading to the Mayor of London, Sadiq Khan, issuing his first “very high” pollution alert.

“The shameful state of London’s toxic air today has triggered a ‘very high’ air pollution alert under my new air quality warning system,” he said in a tweet.

Mr Khan’s office estimates 9,400 premature deaths a year are caused by poor air quality in the capital.

This year London reached its annual limit for pollution in just five days.

London smog: Then and now Show all 18 1 /18 London smog: Then and now London smog: Then and now London A London policeman wearing a mask for protection against the thick fog which hit most of the country and turned to smog in the city London smog: Then and now London A cyclist wears a mask with an air filter as he cycles through Hyde Park London smog: Then and now pollution-UK-2.jpg Heavy smog in Piccadilly Circus, London, 1952 London smog: Then and now London The Shard and St Paul's Cathedral from Hampstead Heath in London London smog: Then and now London A couple of office workers wearing masks to protect them from the dangerous smog in London, 1953 London smog: Then and now London A cyclist wears an anti air pollution mask as she cycles along The Mall London smog: Then and now London A London bus makes its way along Fleet Street in heavy smog,1952 London smog: Then and now London A view of London skyline covered in smog.The environment department confirmed that the air pollution level could reach the top rung on its 10-point scale London smog: Then and now London Morning traffic at Blackfriars, London almost at a standstill because of the blanket smog,1952 London smog: Then and now London Dust settled on a vehicle in South Kensington, which has blown up from the Sahara desert London smog: Then and now London A tugboat on the Thames near Tower Bridge in heavy smog, 1952 London smog: Then and now London A view of the Tower Bridge as the country continues to experience 'very high' levels of pollution London smog: Then and now London A couple wearing masks to protect them from the smog in Blackfriars,1954 London smog: Then and now London Tourists wear face masks London smog: Then and now London A woman wears mask in London, 1953 London smog: Then and now London A woman wearing an anti-pollution mask rides a bicycle at Hyde Park Corner London smog: Then and now London Smog masks have become all the rage in London due to the life threatening levels of air pollution with result in severe smog or 'pea soupers', 1953 London smog: Then and now London A couple stands on the viewing platform of a skyscraper

A text message warning from City Hall sent to people in affected boroughs read: “High air pollution forecast for Thursday. Consider spending less time outdoors, take medication. If unwell contact GP.”