Sadiq Khan's office has been accused of trying to counter scientists who suggested that the London Mayor's ultra low emission zone (ULEZ) policy had little impact on pollution.

Private emails between the mayor's office and an academic were seen by The Telegraph.

In the emails, Shirley Rodrigues, the London Mayor's deputy for environment and energy, told Frank Kelly, an Imperial College professor, to counter research, covered in the media, that questioned the effectiveness of ULEZ.

The research was by Imperial's Department of Civil and Environmental Engineering.

Mr Kelly, a director of Imperial's Environmental Research Group, agreed to issue a statement which said ULEZ had ‘dramatically reduced air pollution in London'.

Peter Fortune, Conservative London Assembly Member for Bexley and Bromley, said Khan's ‘office tried to silence Imperial scientists whose research cast doubt on the Ulez scheme's impact on health'.

In response, a spokesperson for the mayor said: ‘It is right - and standard practice across government - that we commission experts to carry out research to inform the work we do.

‘Frank Kelly and the Environmental Research Group at Imperial are some of the world-leading academic institutions looking at air quality. It is normal and proper to work with these experts to ensure our policies are as effective as possible at dealing with issues such as the high number of deaths – up to 4,000 a year – linked to toxic air in London every year.

‘The ULEZ analysis from the engineering department at Imperial only paints a partial picture, not accounting for the full lifetime impact of the scheme, and only focusing on its immediate impact around its launch.

‘It is commonplace for academic experts to disagree with how other academic studies are interpreted, as was the case here.'

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