European Union regulators have begun their antitrust review Friday of the $26 billion acquisition of Hilton Hotels Corp. by Blackstone Group LP.

The European Commission, the antitrust arm of the EU, said that it notified both companies Friday.

The investigation is procedural and is due to the magnitude of the deal. Hilton is the second-largest hotel chain in Europe, behind only Marriott International Inc.

However, the Brussels-based regulator, which is to rule on the transaction by Oct. 19, has the power to alter or outright nix the deal if it finds reasons to do so.