"It concerns almost the entire watch industry, including the Swatch group," the watchmaker said in a statement.
The European Commission had thrown out the complaint filed by the European Confederation of Watch & Clock Repairers' Associations in 2004, but was ordered to reverse that decision by the European Court of Justice last year.
The confederation alleged that watch manufacturers refused to supply spare parts to repairers that did not belong to manufacturer maintenance networks.
"We are confident regarding the outcome of this investigation," Swatch group said.
Stocks in the watchmaker fell 7.39 percent in late afternoon trade to 335.70 francs per share, leading the Swiss Market Index's 2.98 percent fall, as investors continued to fret over the credit worthiness of the United States and other advanced economies.