The companies announced the deal in May, which will bring together Britain's biggest cable TV provider in Liberty's Virgin Media with Telefonica's O2, the second-largest mobile operator.

The EU competition enforcer can either clear the deal with or without conditions during its preliminary review or it can open a four-month long investigation if it has serious concerns.

Rivals in the British market include Vodafone, Comcast's Sky, Three UK and TalkTalk.

(Reporting by Foo Yun Chee; editing by Barbara Lewis)