Boris Johnson has been accused of trying to silence a group of scientists who drew attention to the severe air pollution problem in London.

Matthew Pencharz, the mayor of London’s environment adviser, complained on Mr Johnson’s behalf to the principal of King’s College London after one of its senior academics said that Oxford Street had the world’s highest level of nitrogen dioxide pollution.

Scientists at King’s, which receives funding controlled by Mr Johnson for its air quality research, believe that Mr Pencharz’s letter contained a veiled threat that the money could be withdrawn.

David Carslaw, principal air quality scientist at King’s environmental research group, told a conference in June that the NO2 concentration recorded by a roadside air pollution monitoring unit in Oxford Street were