BRUSSELS/MADRID (Reuters) - EU antitrust regulators are likely to expand their scrutiny of Orange's (>> ORANGE SA) 3.4-billion-euro (2.6 billion pounds) bid for Jazztel (>> Jazztel PLC), stepping up pressure on the companies to enhance their concessions, two sources familiar with the matter said on Wednesday.

Orange, France's largest mobile firm, is looking to use the deal to get ahead of rival Vodafone (>> Vodafone Group plc) in the Spanish mobile market. Telefonica (>> Telefonica SA) is the market leader there.

The companies offered concessions to the European Commission on Nov. 13. The sources said the EU competition watchdog did not consider these sufficient.

One of the sources said the Commission was likely to announce the opening of an in-depth investigation on Thursday and also reject a request from the Spanish competition authority to examine the case.

Commission spokesman Ricardo Cardoso, Orange and Jazztel declined to comment. The deadline for the Commission's preliminary review is Thursday, Dec. 4. An in-depth probe takes up to 90 working days and can be extended to 105 working days.

Jazztel shares were down 0.8 pct at 12.57 euros at 1515 GMT, the biggest loser in Spain’s blue-chip index Ibex which was up 1.26 pct.

The acquisition of Jazztel would add 1.5 million broadband subscribers to Orange's client base and also help it take on competitors with fixed, TV and wireless packages.

The Spanish telecoms market has seen a wave of consolidation in recent months as tough competition and falling prices hit operators. The most notable deal was Vodafone's 7.2-billion-euro takeover of Spanish cable operator Ono in March.

(Additional reporting by Gwénaëlle Barzic in Paris; editing by Julia Fioretti/Keith Weir)

By Foo Yun Chee and Julien Toyer

Stocks treated in this article : ORANGE SA, Telefonica SA, Jazztel PLC, Vodafone Group plc