Sir Keir blamed the Mayor’s ULEZ expansion for the loss in Uxbridge and South Ruislip on Thursday, claiming it had handed victory to the Tories.
Speaking on a visit to Shefford in Bedfordshire, Sir Keir told broadcasters that there was “no doubt” that the party lost the by-election because of the Ulez expansion.
“I have said we should reflect on this, including the mayor. I have spoken to him as you would expect, and so there will be that reflection.”
Addressing representatives at the National Policy Forum - which will help shape the policies which end up in Labour’s manifesto - Sir Keir went further in criticising the scheme.
He said: “That result in Uxbridge demonstrates there is never any reason to be complacent and never a reason to rest on our laurels.
“We’ve got to ask ourselves seriously - are our priorities the priorities of working people or are they just baggage that shows them we don’t see the country through their eyes?”
Mr Khan has sought to signal he is in listening mode and it is understood from those close to city hall that he is happy to look at any new ideas for ways to mitigate the impact on Londoners.
However, sources told Sky News that he will not compromise on any moves that could reduce the effectiveness of the policy.
A source close to the Labour mayor said: “Sadiq has been clear he is listening to Londoners following this by-election.”
Mr Khan’s team defended his plan, saying only one out of 10 cars driving in outer London would face the charge, with a £110 million scrappage scheme available to help lower earners to upgrade their vehicles.
ULEZ is due to expand at the end of August from the boundary of the North and South Circular roads to the whole of Greater London, requiring drivers of the most polluting vehicles to pay an additional £12.50 a day.