The Competition and Markets Authority (CMA) announced that the action was taken because the companies did not provide any assurances to allay its worries that, should the plans proceed, mobile phone consumers will have to pay more and receive worse service.
The largest mobile network in the UK would be created by the proposed £15 billion merger, which was revealed last year and would unify 27 million users under one carrier.
Although the regulator had earlier stated that it had not found any evidence that the deal would be good for competition or investment, concerns have been expressed that it will lessen rivalry amongst operators.
The corporations were given five working days by the CMA last month to provide “meaningful solutions” to its concerns, failing which they would be subject to a protracted, in-depth “phase two” examination.
