Samsung and Philips have revealed that they were among several consumer electronics companies raided recently by European Commission inspectors.
Thursday's raid focused on companies making and selling consumer electronics products and small domestic appliances.
The Commission said it was concerned these firms had colluded in restricting online availability of some products in a bid to restrict competition.
Media-Saturn, the European consumer electronics retailer, was also raided.
Both Samsung and Philips said they were co-operating with the authorities.
"The Commission has concerns that the companies concerned may have violated EU antitrust rules that prohibit anticompetitive agreements or concerted practices," it said in a statement.
Restricting supply "may lead to higher consumer prices or the unavailability of products through certain online sales channels", it said.
The Commission did not give details of the companies it had visited.
If found guilty, the companies could face fines equivalent to 10% of global annual turnover.
In Samsung's case, this could amount to £17bn, although a fine of this size would be unprecedented and extremely unlikely.
Sources familiar with the case told the BBC that agreements between manufacturers and retailers are not necessarily illegal.
Luxury goods brands, for example, often want to restrict the supply and distribution of their products to safeguard their exclusive appeal.
Whether such a defence would apply to low-cost electronic products is for the lawyers to decide.
"The fact that the Commission carries out such inspections does not mean that the companies are guilty of anticompetitive behaviour," the Commission said, "nor does it prejudge the outcome of the investigation itself."
EU competition commissioner Joaquin Almunia has been stepping up antitrust investigations under his watch.
This week the Commission fined eight banks - including Royal Bank of Scotland - a total of 1.7bn euros (£1.4bn) for forming illegal cartels to rig interest rates in the financial derivatives markets.
And in May, Commission inspectors raided several oil companies suspected of price-fixing.