"The OFT's initial view, following consultation, is that the joint venture threatens significantly to affect competition in mobile telecommunications in the UK," the regulator said in a statement.
"If the request is granted, the OFT intends to examine the proposed joint venture with a view to deciding whether it should be referred to the Competition Commission for an in-depth investigation."
The OFT is partly responding to pressure from UK consumer group Which? which claims the deal could stifle competition.
However, the respective parents of the two networks, France Télécom and Deutsche Telekom, have long claimed that the proposed 50:50 joint venture would control a significantly smaller slice of the market - about 37% at current levels - than in many other European countries, where it is common for one operator to command market share of over 40%.
An investigation by the OFT would also massively delay the deal, which was slated for initial EC approval this month.