Sadiq Khan has spent £150million on technology that allegedly will enable the introduction of pay-per-mile road charging on motorists.
Transport for London has created "Project Detroit" has a total of 157 staff are now working exclusively on the scheme.
A Freedom of Information request showed some engineers were paid more than £100,000 a year.
The scheme aims to have a single "road user charging" platform for the congestion charge, Ultra Low Emission Zone (Ulez) and Low Emission Zone.
Sadiq Khan's Ulez expansion has proved controversial
PA
Conservatives have accused that Project Detroit could be used to introduce a charge based on the distance driven in vehicles within London. TfL has denied these claims.
One FoI response from TfL read: "The Detroit platform has the capability to be extended and we will be looking to build the system flexibly so that other forms of charging based on distance, vehicle type, etc could be catered for if a decision was made in future to do so."
London Assembly member for Bexley and Bromley, Peter Fortune said: "Sadiq Khan can deny it all he wants but it's pretty clear he plans to introduce pay-per-mile road-user charges for every motorist if he wins a third term."
Around £21million has already been spent on the scheme, which began in 2021, while the predicted final cost is believed to be between £130million and £150million.
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The scheme has proved controversial
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A TfL spokesperson said: "Any work carried out or staff hired as part of Project Detroit has been in relation to TfL's existing road-user charging schemes.
"This was part of TfL's wider work to bring in-house the currently outsourced system for which the contract expires in 2026.
"Pay-per-mile charging has been ruled out by the mayor and no such scheme is on the table or being developed."
It has been thought that older vehicles which pollute the environment and motorists using roads at peak times could be charged more under the scheme.
Ministers told the Commons in July last year that Khan was planning to hit drivers with a pay-per-mile or pay-per-minute regime to use the capital's roads. This accusation was denied by TfL.
The Congestion Charge operates in central London, while the Ulez, LEZ and DVS cover all of Greater London.
London Mayor Sadiq Khan made the decision to expand the Ulez in August, despite opposition from drivers, residents and politicians.
Within the first two months, around 60,000 vehicles had been hit with a £12.50 charge.