Investigators from the DGCCRF fraud squad raided several sites last Thursday, Renault admitted after a leaflet by the CGT union gave details of the raids and
suggested they were "linked to the consequences of the Volkswagen rigged-engines affair".
Personal computers belonging to several directors were seized, the union said.
Representatives of another union, the CFDT, said they had asked management about the raids on Wednesday but received no answer.
Renault shares slumped as news of the union claim spread and shares in the rest of the automobile sector, notably those of PSA Peugeot-Citroën, followed.
Later in the day Renault confirmed that the raids had taken place as part of an investigation into French carmakers ordered by Environment Minister Ségolène Royal following last year's revelations that Volkswagen had rigged engines to dodge emissions controls in the US.
But it said that the ministry had concluded that the tests seemed to find no evidence of rigging software.
Renault pledged last month to invest 50 million euros to bring its cars' real emissions in line with those measured in official test conditions.
Peugeot, whose share value fell seven per cent on Thursday, announced that there had been no fraud raids on its facilities and said government tests had found no problems.