The ultra low emission zone introduced by Sadiq Khan, the Mayor of London, is “not a silver bullet”, academics have claimed after a study found it made only a small difference to air quality in the weeks after being introduced.

Researchers at Imperial College London studied air quality in 12 weeks covering the period before and after the zone was introduced in April 2019, finding it contributed “only marginally” to air quality improvements.

The zone means the drivers of older, more polluting vehicles must pay a fee to drive in some areas of the capital.

Dr Marc Stettler, from Imperial’s department of civil and environmental engineering and centre for transport studies, said the study showed ultra low emission zones “contribute only marginally to cleaner air”.

“Our research suggests that an ultra low emission zone on its own is not an effective strategy to improve air quality – the case of London shows us that it works best when combined with a broader set of policies that reduce emissions across sectors like bus and taxi retrofitting, support for active and public transport, and other policies on polluting vehicles,” he said.

Last month, the London zone was extended to cover more of the capital, while similar schemes have been introduced in Bath, Birmingham and Glasgow.