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The EC has until 1 March to make a decision.
The OFT said if the request is granted, the matter will be referred to the UK Competition Commission for thorough investigation.
The joint venture threatens to affect competition in mobile telecommunications in the UK, the OFT said. The number of mobile operators in the UK would be reduced from five to four.
The merger between Orange and T-Mobile would create the UK's largest mobile operator, with around 29 million customers, or a 37% share of the market.
France Telecom and Deutsche Telekom have said there is no case for concessions to secure regulatory approval, but the OFT investigation could force a rethink, according to the Financial Times.
Consumer groups have lobbied the OFT against the proposed merger, fearing a loss of choice and competition, while rivals have voiced concerns that the new company will hold most of the radio spectrum in the 1,800MHz range, giving it an unfair advantage in providing mobile data connections.
A merger would also cause job losses among the combined workforce of 19,000. Headquartered in Bristol, Orange employs 12,500 in the UK, while Hatfield-based T-Mobile employs 6,500.