Transport for London (TfL) generated more than £90m last year from expanding the capital's Ultra Low Emission Zone.
But the Mr Khan insisted he wouldn't "pre-judge" the issue and make a final decision, until he had received a report from Transport for London by Christmas.
Nicholas Lyes, the RAC's head of roads policy, said: "While we accept that action is needed to reduce toxic emissions from vehicles, the cost-of-living crisis is hurting drivers in the pocket and there is a risk that further enlarging the zone will be hugely costly for those with older vehicles, who can least afford to change them for something newer.
However, Mr Khan said "the status quo in relation to the poor quality of air in London is just not on".
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