Fears that merging mobile phone operators Orange and T-Mobile will damage UK market competition have prompted the Office of Fair Trading (OFT) to request an investigation.

If the deal goes ahead it will create the UK's biggest mobile operator, with 28.4 million customers and a 37% market share between Deutsche Telekom's T-Mobile and Orange, owned by France Telecom. The combined organization would also have a much larger potential presence in the mobile broadband and home broadband markets.

The economic regulator wants the European Commission to let it examine British implications of the merger, and said: "The OFT's initial view, following consultation, is that the joint venture threatens significantly to affect competition in mobile telecommunications in the UK."

Brussels is expected to decide whether to investigate itself or refer the matter to the OFT within 10 days, and if does approve the request, OFT will have the option of asking the Competition Commission for an in-depth review.

This may force the two companies to make key concessions to satisfy regulators, and will probably involve a drawn-out period of uncertainty.

Since the deal was announced last year consumer groups have lobbied the OFT in protest.

Rival mobile firms said the merged operator will dominate the 1800 megahertz range of the radio spectrum, which enables faster mobile broadband speeds on wireless technology.