Transport for London (TfL) has announced its 9,000-strong core bus fleet now meets or exceeds the cleanest Euro VI emissions standards.
The move is forecast to see nitrogen oxide emissions and particulate matter from buses fall by an average of 90% and 80% respectively, according to the organisation.
The government body, which is responsible for the transport system in London, started phasing out diesel buses and retrofitting older buses in 2017.
Since then, an estimated £85 million investment has been made to upgrade the bus fleet.
More than 400 all-electric buses have been introduced and around 300 additional zero-emission buses are expected to join the fleet by the end of this year.
There are also plans for 2,000 all-electric buses to be in operation by 2025.
The Mayor of London Sadiq Khan said: “Today, as part of our work to tackle the twin dangers of toxic air pollution and the climate emergency, I am proud that the entire TfL core bus fleet now meets the strictest emissions standards.”
Claire Mann, Director of Bus Operations at TfL, said: “Londoners deserve to breathe clean air. Toxic air kills, reduces life quality and, as the pandemic has shown, leaves people more vulnerable to other threats.
‘We’ve already seen how much-retrofitted buses can clean up the air in busy London hotspots, and now with our retrofit programme complete on our core fleet the whole of the city will benefit from improved air quality.”