Boris Johnson is under pressure to take drastic action on air pollution, after scientists discovered that London's Oxford Street is now the most polluted street in the entire world.
The mayor has previously dismissed air pollution levels in London as "perfectly fine".
However, monitoring stations on London's Oxford Street have discovered levels of Nitrogen Dioxide (NO2) at up to ten times the legal limit.
Scientists at King's College London said levels of the gas, caused by diesel vehicles, were now even higher than those found in Beijing.
"To my knowledge this level is the highest in the world in terms of both hourly and annual mean," David Carslaw, who researches vehicles emissions and air quality at King's College London, said.
"NO2 concentrations in Oxford Street are as high as they have ever been in the long history of air pollution."
The toxic gas causes breathing difficulties and has been linked to increases in premature deaths and hospital admissions.
Across London, around 4,000 deaths a year are attributable to air pollution.
A study into the development of children's lungs in East London, also found evidence that they are failing to grow properly due to air pollution.
Johnson has come under increasing pressure to tackle the problem of air pollution in London.
"The Mayor’s delays and dithering on combating air pollution in London means that not only more people will die prematurely or be seriously affected, but visitors and tourists could now be deterred from visiting Oxford Street," Green Party London Assembly member Jenny Jones said today.
Johnson has been urged to implement temporary closures of central London to traffic, in order to deal with episodes like the recent so-called 'Saharan smog'.
The mayor has so far dismissed these calls and has also set out plans for a wave of new road-building projects in London, including a new inner ring-road and a series of crossings of the Thames in East London.
The plans have met opposition from local people concerned about existing high levels of pollution in East and South East London.
Last month Johnson gave in to pressure to appear before a parliamentary inquiry on air pollution.
He is now set to give evidence to the Environmental Audit Committee in September.