LONDON-- The U.K.'s antitrust authority, the Competition and Markets Authority, said Wednesday it is considering scrutinizing Asian conglomerate's Hutchison Whampoa Ltd.'s proposed acquisition of Spanish telecommunications giant Telefonica SA's (TEF.MC) U.K. mobile operator O2.

The CMA said it may request European Union competition authorities to refer the case back the U.K. body for an in-depth inquiry, it said in a statement. The CMA said it is seeking industry comments on the matter until Sept. 24, including concerns that the merger of Hutchison's U.K. mobile operator Three and O2 would result in a "substantial" lessening of competition in the U.K.

Last week, the EU said it would decide by Oct. 16 whether to clear the deal, which could face resistance from the bloc's regulators as the continent's consolidation of scattered telecom operators gathers pace.

Scandinavian telecom operators Telenor ASA (TEL.OS) and TeliaSonera AB (TLSN.SK) abandoned plans to combine their Danish operations after failing to secure European antitrust approval, possibly indicating a tougher approach from EU regulators toward mergers that reduce the number of mobile-phone operators in individual European countries from four to three.

At the end of March, Hutchison reached a deal to acquire O2 for about $15 billion, in a transaction which might help the company to focus attention on its primary growth markets of Latin America.

The deal for Hutchison would seal Hong Kong billionaire Li Ka-shing's telecom and infrastructure company as one of Europe's top wireless providers.

The acquisition would more than triple Three's U.K. subscribers to 34 million and create the country's biggest mobile operator, leapfrogging EE--owned by Orange SA (ORA.FR) and Deutsche Telekom AG (DTE.XE)--which has 28 million, and leaving Vodafone Group PLC (VOD.LN) at the bottom with 20 million.

Hutchison, whose holdings include telecom assets across Europe and Asia as well as ports and energy companies, said in January that it was in exclusive talks to buy O2.

The move came swiftly after BT Group PLC (BT.A.LN), the U.K. telecom incumbent, chose EE over O2 to potentially cement a near-$20 billion tie-up. BT's ambition to create a telecom powerhouse of more than 30 million customers sparked a frenzy of war-gaming across the U.K.'s telecom and media sectors as firms bid not to be left behind.

The CMA is screening BT's proposed deal for EE. It said the transaction may result in less competition given that they are the largest suppliers of fixed and mobile communications services in the U.K.

BT says the deal would be good for investment in the industry and bring consumer choice.

-0---Write to Simon Zekaria at simon.zekaria@wsj.com 
 

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September 16, 2015 11:45 ET (15:45 GMT)