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 breach legal limits of NO2 there needs to be a recording of 18 hours of at least 200 micrograms per cubic metre at any individual site.
By Monday, London's NO2 hourly average had exceeded the limit for only up to eight hours.
"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 can increase the likelihood of respiratory and cardiovascular diseases.
'Diabolical to terrible'
Friends of the Earth clean air campaigner Oliver Hayes said: "The windy start to 2018 is likely to have had as much to do with the reduced number of pollution spikes as anything else, but it's clearly good news for Londoners that their air isn't as filthy as usual this January.
"However, with the entire year's quota of pollution breaches due to be used up within the month, London's pollution can at best be downgraded from diabolical to terrible.
"The Mayor has introduced several good initiatives but much more needs to be done."