The merger of the two companies, announced in September, could face further delays because the OFT may inform the Commission of concerns over the development and call for further scrutiny.
The merger will create the UK's largest mobile phone operator, with a 37 per cent market share. The OFT is understood to be worried about the new entity monopolizing mobile phone spectrum.
Orange and T-Mobile's parent companies, France Telecom and Deutsche Telekom, respectively, have said they see no need for concessions to get approval for the 50-50 joint venture from the regulators.
The expected OFT announcement will be welcomed by consumer groups which pressed the Commission to allow the British regulator to examine the deal over fears that the merged entity could drive up prices and hamper competition.
The OFT's decision could be followed by an investigation by the Competition Commission, whose investigations can run for six months or more.