The OFT said its initial view is that the joint venture “threatens significantly to affect competition in mobile telecommunications in the UK”.
If its request is granted by the EC, the OFT it will examine if the merger should be referred to the Competition Commission for an in-depth investigation.
The merger of France Telecom’s Orange network and Deutsche Telekom’s T-Mobile would create the largest mobile phone operator in the UK, with around 29.5m customers.
The combined network would have a market share of 37%, ahead of O2, which has a 25% share and Vodafone, which has 27% of the market.
The network operators refered the merger deal to the EC but consumer groups have called for it to be investigated by the UK copetition authority.
Media watchdog Ofcom has previously warned that the merger between Orange and T-Mobile could trigger an in-depth review of the UK’s mobile market.
The deal was formalised in November, two months after it was announced.