Residents who are entitled to a 90% discount on the London congestion charge may lose it if the scheme is extended.
Traffic levels have been reduced by about 15%
London mayor Ken Livingstone has said he would like to extend the zone into Westminster and Kensington and Chelsea.
Derek Turner, formerly of Transport for London (TfL) and the man behind the scheme, told the Times that a discount for a larger number of people would reduce the effectiveness of the scheme by allowing more cars on the road.
The £5-a-day charge, which came into effect on 17 February, has reduced traffic in central London by about 15%.
According to the newspaper, Derek Turner told a meeting of city leaders discussing the charge: "We may have to change the discount rate because there would be more residents driving in an enlarged zone and that would reduce the decongesting effect."
But Westminster Council deputy leader Kit Malthouse said he feared that would discourage people from living in parts of London.
He said it would turn parts of London into a "ghetto of the very rich or the very poor".
"The people who would get squeezed would be young families who depend on their cars and elderly people who find public transport difficult."
TfL said there were no firms plans yet to extend the scheme although it was looking into the technical feasibility.
It said it was also looking at a separate congestion charge scheme around Heathrow Airport although nothing would happen before next year's mayoral election.
Ken Livingstone said: "I have read with interest the reports that I may abolish the 90% residents' discount if the scheme is extended to cover a wider area.
"I can give an assurance that I have no such plans to reduce or abolish the residents' discount."