Thousands of speeding offences could be overturned due to the font used on variable speed limit signage not complying with traffic regulations.
The signs, which appeared on a stretch of the M42 west of Coventry, are said to have shown mph numbers that were taller and narrower than they should have been.
According to the BBC, The CPS alerted Warwickshire Police last November to the irregularity of the signs.
Some lawyers and traffic consultants now want any penalties that 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 police have taken the decision to stop using the signs to enforce speed limits and will drop any prosecutions they were intending to pursue regarding offences on this stretch of road.
However, thousands of motorists have 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 and nine of the M42 last year.
Speaking to the BBC, Richard Bentley, a traffic management consultant and former police officer, said: "There should be a situation where cases are opened in the magistrates court to have the cases reheard and the convictions quashed.
"If there are no traffic signs, the Act of Parliament prohibits the conviction - and these are definitely not traffic signs."