By Foo Yun Chee and Nerijus Adomaitis

"The Commission has concerns that the inspected companies may have violated EU antitrust rules that prohibit cartels and restrictive business practices," it said in a statement.

While the Commission did not name the companies involved, Mowi, the world's biggest producer of farmed Atlantic salmon, told Reuters that the EU regulators had raided two of its businesses, in Scotland and the Netherlands.

"We have nothing to hide, we are cooperating with the European Commission," Mowi spokesman Ola Helge Hjetland said. Mowi, formerly known as Marine Harvest, changed its name in January as part of a rebranding.

Norway's Grieg Seafood said that one of its facilities in Scotland's Shetland Islands had been inspected.

"Grieg Seafood aims to be open, transparent and forthcoming and will provide all necessary information requested by the European Commission DG Competition in its investigation," it said in a statement.

The Chief Financial Officer of Norway's Salmar, Trond Tuvstein, said that the Commission had raided Scottish Sea Farms, its joint venture with Leroey Seafood.

He said the company was also cooperating. Leroey was not immediately available for comment.

Among other major salmon companies, Norway Royal Salmon said it was not affected.

The Commission said its officials carried out unannounced inspections in several member states and were accompanied by their counterparts from national competition authorities.

It did not name the countries. Norway is not an EU member and the Norwegian competition authority was not involved in the EU's investigation, its spokeswoman Margrethe Gudbrandsen said.

The Commission said that an inspection was a preliminary step in an investigation and did not mean that the companies were guilty of anti-competitive behaviour nor did it pre-judge the outcome of the investigation itself.

Companies found guilty of breaching EU antitrust rules can be fined up to 10 percent of their global turnover.

The Commission said there was no legal deadline to complete the investigation.

(Reporting by Foo Yun Chee in Brussels and Nerijus Adomaitis and Terje Solsvik in Oslo; writing by Alister Doyle; editing by Francesco Guarascio/Alexander Smith/Jane Merriman)