In revolt, several councils surrounding the capital have refused to sign an agreement allowing Transport for London (TfL) - of which Mr Khan is chairman - to install street signs and cameras warning drivers they are approaching the clean air zone.
Mr Khan said earlier this week that he has asked TfL chiefs to look into developing a scheme which would use more “sophisticated technology” to charge road users, according to the Telegraph.
Answering a question from the London Assembly, he said: "ANPR (automatic numberplate recognition) cameras could form part of the potential operation of such a scheme but no proposals have been developed."
Mr Khan has previously hinted that he is looking to adopt a system similar to Singapore, where sensors on gantries monitor cars’ movements and charge more for travelling at peak times.
Christina Calderato, TfL’s strategy director, told an Assembly transport committee this week that TfL was keen to introduce charging at the "earliest possible stage" but said a dialogue with Londoners is needed.
A spokesperson for the Mayor of London said: “TfL has outlined how one future option could be to abolish existing charges and replace them with a single simpler road user charging scheme which could take into account factors such as local public transport availability, employment and income, but the technology required is still many years away.
“Any formal proposals which could be developed in the future would be subject to consultation with information provided on detailed scheme proposals and their likely impacts.”
The ULEZ scheme charges older polluting vehicles - typically pre-2015 diesel cars and pre-2006 petrol vehicles - for driving within the zone in a bid to encourage motorists to buy newer, less polluting cars.
Mr Khan is pressing ahead with the plans despite around 60 per cent of respondents opposing it in a consultation. He says the expansion will prevent 27,000 tons of CO2 being emitted in outer London.