London's air quality is within legal limits in mid-January for the first time in 10 years, City Hall has said.
The capital breached limits for nitrogen dioxide (NO2) by 6 January every year for the last decade, Mayor Sadiq Khan said.
So far this year, London's NO2 has not exceeded limits – although it is likely to later this month, Mr Khan admitted.
The Department for Environment, Food and Rural Affairs said despite recent progress, there was still more to do.
"While we have come a long way since 2010, we recognise there is more to do which is why we have put in place a £3.5bn plan to improve air quality and reduce harmful emissions", a Defra spokesman said.
"We will also end the sale of conventional new diesel and petrol cars and vans by 2040."
A recent NO2 study found Marylebone Road and Hyde Park Corner were the most polluted postcodes in Britain.
For London to exceed the legal limit for NO2 emissions, there needs to be a recording of 18 hours of at least 200 micrograms per cubic metre anywhere in the capital.
"At long last we are seeing some improvements in our toxic air. But I can't stop this health crisis without more help", he said.
"Londoners deserve a government that wakes up to the scale of this problem and delivers tough new air quality legislation so that legal limits are met all year round."
Other data released last year revealed every area in London exceeded World Health Organisation (WHO) limits for PM2.5.
PM2.5 particles which can increase the likelihood of respiratory and cardiovascular diseases.