Thousands of motorway speeding convictions could be overturned because the font used to display the numbers on some variable speed limit signs may not have complied with traffic regulations.
The Crown Prosecution Service said the signs showed mph numbers taller and narrower than they should have been.
The signs are used to reduce congestion on busy stretches of motorway.
The CPS alerted Warwickshire Police last November of the irregularity of the signs on the M42 west of Coventry.
Some lawyers and traffic consultants now want any penalties which were handed out over the course of the six years the signs were in place to be quashed, arguing they are not legally enforceable.
The Highways Agency, which installed the signs, said it believed they were the right size and were clearly visible to motorists.
The signs were on two stretches of the M42, between junctions 3a to seven and junctions seven to nine.
Cases dropped
As a result police took the decision to stop using the signs as a means of enforcement and dropped prosecutions it was intending to pursue on the stretches of road affected.
However, by then thousands of motorists had already received fines and convictions built up since the first of the signs went into operation in 2006.
At least 11,000 fixed penalties were issued to motorists breaking the variable speed limit between junctions seven to nine of the M42 last year.
