French meat processing company Spanghero knowingly sold horsemeat labelled as beef, the French government has said.
Spanghero's licence is being suspended while a probe continues, agriculture minister Stephane Le Foll said.
The firm has denied the allegations, saying it only ever dealt in meat it believed to be beef.
A widening scandal over mislabelled horsemeat has affected at least 12 European countries.
Spanghero imported meat from Romania and sold it on to another company, Comigel, which made frozen ready meals at its factory in Luxembourg.
French consumer affairs minister Benoit Hamon said the meat left Romania clearly and correctly labelled as horse. It was afterwards that it was relabelled as beef.
"From the investigation, it would seem that the first agent or actor in this network who stamped 'beef' on horsemeat from Romania was Spanghero," Mr Hamon said.
There was "no reason to doubt the good faith" of the Romanian abattoir that originally provided the meat, Mr Hamon added.
The French government believes that the 'fraud' went on for six months and involved around 750 tonnes of meat.
The investigation says some blame may rest with Comigel, which made the ready meals sold around Europe.
Staff there should have noticed anomalies in the paperwork, and realised from the smell and look of the meat once it was defrosted that it was not beef, Mr Hamon said.
Spanghero has strongly denied wrongdoing.
"Spanghero confirms having placed an order for beef, having been led to believe it received beef, and having sold back what it thought was beef, properly labelled as such, in line with European and French regulations," the company said in a statement.