London Mayor Boris Johnson is being urged to cut diesel emissions “as soon as possible” to clean up the capital’s air quality.

The London Assembly, an elected watchdog which scrutinises the Mayor’s plans and programmes, claims the capital’s air quality is among the worst in Europe.

Diesel exhaust is said to be a major contributor to air pollution, with road traffic responsible for 40% of London’s emissions of nitrogen oxides (NO₂).

The Assembly’s Environment Committee has made a number of recommendations which include the Mayor’s plans to introduce the Ultra-Low Emission Zone (ULEZ) before 2020 to be expanded and brought forward.

It also suggests the Mayor should “work closely” with the boroughs and government to learn how London could achieve compliance with European air pollution limits by the end of the decade.

In April, the Supreme Court ordered the UK Government to take immediate action to tackle the dangerously high levels of nitrogen dioxide in the country.

Stephen Knight AM, Environment Committee Member said: “We urge the Mayor and the government to take our recommendations on board and we call on the Mayor to finally take ownership of the matter in order to help London’s air quality meet legal limits.

“As petrol engines become cleaner with time it’s becoming clear that diesel emissions are a large part of the problem.”

A spokesperson for the Mayor added: “The Mayor will continue to lobby the European Union to implement effective on-road testing of diesel vehicle emissions and the government to use fiscal incentives to encourage only the cleanest vehicles such as providing funding to scrap the most polluting diesel vehicles.”

The use of cleaner buses on London roads as well as age limits on polluting taxis have helped “halve the number of Londoners living in areas above nitrogen dioxide limits, brought nitrogen oxide emissions down by 20% and emissions of particulate matter down by 15%”, the spokesperson said.