Measurements recorded at Brixton Road in south London's Lambeth area showed pollution levels surpassed hourly limits for nitrogen dioxide concentrations with readings in excess of the European Union limits.
Its director, Simon Birkett, said that "with diesel vehicles responsible for 90-95 percent of NO2 from vehicle exhaust", the only way to comply with the law was to "ban diesel from the most polluted places".
This year's first breach in London was on the main A23 road running through Brixton in South London.
This week new data also revealed that modern diesel cars produce 10 times more NO2 pollution than heavy trucks and buses per litre of fuel, which experts say is due to the much tougher testing faced by heavy vehicles.
His immediate plans include a new "Toxicity Charge" in the Congestion Charging Zone later in 2017 and introducing the world's first Ultra Low Emission Zone, which may be delivered as early as 2019 and cover an area over 18 times larger than was originally planned.
While the mayor of London, Sadiq Khan, is pledging additional resources to fight this problem, the government's proposals have twice been ruled illegal by the courts and measures to reduce air pollution have stalled. Yet, despite the key culprit in the United Kingdom being well known - diesel vehicles - the government has been asleep at the wheel for years.
But the issue is not just confined to London.
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Road transport is the biggest problem for air pollution and diesel vehicles are the worst of all. Most air quality zones across the country break legal limits and the crisis was called a "public health emergency " by MPs in April. While these are vital steps in the right direction, we can't wait another three years for action.
One such measure was announced by the incumbent mayor of the city, Saqid Khan, who has promised to pledge almost £900m over the course of the next five years to reduce the harm that leads to almost 6,000 deaths annually in London.
Jenny Bates, a campaigner at Friends of the Earth, said in a statement.
The mayor has pledged to double funding to tackle air pollution to £875m over five years.
"It's a public health catastrophe and needs an urgent response". "This is why the government must take much bolder and quicker action including planning to phase out diesel by 2025".
ClientEarth also said the "perverse" financial incentives that encourage people to buy diesel cars rather than cleaner ones must be ended.