Several council areas that neighbour London have refused to warn motorists if they are about to drive into the newly expanded ultra low emission zone (Ulez).
Six out of the seven Home Counties surrounding Greater London have not signed an agreement with Transport for London (TfL) to install warning signs, the BBC reported.
It means motorists will not be told when they are driving into the newly expanded zone, meaning those without compliant vehicles that meet minimum emissions standards could be fined.
London mayor Sadiq Khan's expansion, which came into effect on Tuesday, imposes a daily charge of £12.50 on vehicles entering Greater London.
Conservative-led Kent, Surrey, Buckinghamshire, Hertfordshire, Essex and Thurrock councils have all refused to allow warning signs to be erected that tell drivers they are approached the Ulez.
Only Slough, the smallest local authority just outside the new zone, has reached an agreement with TfL.
The RAC said the councils' refusal to agree to signage would leave motorists in the dark about entering the Ulez zone.
Watch: £12.50 daily charge introduced as Ulez expands across whole of London
Spokesperson Simon Williams told Yahoo News UK: “While it might appear valiant for several councils around London to stand up to the mayor in this way, it’s likely to mean drivers don’t get any advanced warning of where the new Ulez boundary begins.
"They will then end up having to pay the charge rather than having a chance to avoid the area in the first place. As the High Court has ruled that the ULEZ expansion can go ahead, it’s probably time for councils and the mayor to work together rather than against one another.”
A spokesperson for the mayor of London told Yahoo News UK: “All the signs needed for the enforcement of the scheme are in place and the boundaries clearly signposted for drivers.
"The signage and level of information in the public domain means TfL expect drivers to be aware of the standards and the boundary.
“Highways England has co-operated fully and responsibly with TfL, meaning there is Ulez signage on major roads leading to the London boundary and at all points of entry to the zone. These councils should do the same, and do what’s best for their residents and drivers.”
At the end of last month, the High Court dismissed a challenge by five councils - outer London boroughs Bexley, Bromley, Harrow and Hillingdon - alongside Surrey Council - against the Ulez expansion.
Khan, who has faced strong opposition to the expansion, insisted on Tuesday that the scheme was not "anti-car", claiming that nine out of 10 cars in outer London are already compliant.
Separate figures obtained by the RAC show more than 690,000 licensed cars in the whole of London are likely to be non-compliant.
This does not take into account other types of vehicles or those which enter London from neighbouring counties.
However, the mayor's office claimed the RAC data is not accurate.
A TfL spokesperson told Yahoo News UK: "We have an extensive camera and signage network which is sufficient to support the effective operation of the scheme.
"Anyone driving a non-compliant vehicle within the expanded zone will be detected, and we advise everyone to check whether their vehicle is compliant and to consider the various support that is available.”
Roger Gough, leader of Kent Council, told the BBC: "We're not prepared to facilitate a scheme against the interests of our residents and for which the mitigation others in London have been offered, have not been offered."
Khan has extended the scrappage scheme to all Greater London residents with non-compliant vehicles, but there is no support for those living just outside the newly expanded zone who regularly commute towards the capital.
Watch: Transport secretary criticises Ulez expansion