ULEZ: Does Starmer backs mayor or Labour candidate on air pollution fee?
The Labour leader said both calls to extend and delay the Ultra Low Emission Zone (ULEZ) to outer London "need to be accommodated".
London's mayor Sadiq Khan wants to the expand the current zone from inner London to the much wider Greater London boundary.
But the party's candidate in Uxbridge and South Ruislip, to succeed Boris Johnson, wants the extension delayed after protests from residents and businesses.
BBC political editor Chris Mason asked Sir Keir, several times, who was correct and which side of the argument he backed.