The EU executive, which acts as the competition regulator across the 27-country EU, said the complaint related to agreements to delay the entry of cheaper medicines to the market -- so-called pay-for-delay deals.

"The Commission examined whether there may have been violation of EU competition law by GlaxoSmithKline," it said, adding the case had focused on possible abuse of "a dominant position and/or entering into anticompetitive agreements or concerted practices in order to delay or exclude generic competition."

The Commission had said on Thursday it had halted a separate investigation into Anglo-Swedish drugmaker AstraZeneca and Swiss peer Nycomed after failing to find evidence of illegal pay-for-delay agreements.

The regulator can fine companies up to 10 percent of global turnover for breaching EU rules.

Regulators on both sides of the Atlantic have been looking into "pay-for-delay" deals in the pharmaceuticals industry.

• A Permanent Economic Emergency