French rail company SNCF has spent £12 billion on 2,000 new trains that are too wide for many regional platforms, national rail operator the RFF has confirmed.
Around 1,300 platforms out of 8,700 will now need to be modified.
First reported by a French satirical magazine, the error came about because the RFF gave the wrong dimensions to the SNCF.
The RFF measured platforms built less than 30 years ago, overlooking the fact that many of France's regional platforms were built more than 50 years ago when trains were narrower.
Remedial work totalling £40m has already been spent to get the procurement back on track, SNCF said.
It added the trains "were wider to meet public expectations" and that the alterations would cost just 1.25% of its annual maintenance and infrastructure budget.
French transport minister Frederic Cuvillier said an "absurd rail system" caused the error.
"When you separate the rail operator from the train company ... this is what happens.”
SNCF said work will be finished by 2016 to coincide with full delivery of the £12 billion contract.