The planned merger between Vodafone and Three has been thrown into doubt after the competition watchdog said it would launch a major investigation into the Vodafone-Three deal.

The Competition and Markets Authority (CMA) said the move came after the companies failed to provide any measures to allay its concerns that mobile phone users may face. Prices rise, service quality declines If the plan goes ahead.

Proposed £15bn merger, announced last yearwill bring 27 million customers under one provider and create the UK’s largest mobile network.

However, there are concerns it will reduce competition among operators, and regulators have previously said they had seen no evidence that the deal would be beneficial to competition or investment.

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Last month, the CMA gave the companies five working days to come up with “meaningful resolutions” to its concerns or face lengthy, in-depth “stage two” investigations.

But the regulator said in a statement on Thursday that Vodafone and Three had “notified the CMA that they would not provide any commitments”.

The investigation, overseen by an independent panel, will last at least 24 weeks and could result in the merger being blocked.

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Vodafone and Three believe the deal will allow them to increase investment and better compete with main rivals BT/EE and Virgin Media-O2.

The companies said in a joint statement: “This is the expected next step in the process and is consistent with the completion timetable we set from the outset.

“Vodafone UK Three UK remains confident that the deal will drive greater competition in the mobile space and deliver significant changes to network quality, speed and coverage for customers and businesses from day one. “

They added: “We will review the potential issues raised by the CMA and look forward to continuing to engage constructively with them throughout the review process.”

this China Manufacturers Association Three previously said it was “overall the cheapest of the four mobile network operators” but warned this could be under threat as the merger could “reduce competition among mobile operators to win new customers”.

The regulator is further concerned that the deal may make it difficult for smaller “virtual” operators such as Sky Mobile, Lebara and Lyca to negotiate good deals for their own customers, as it will reduce the number of network operators available to host these customers quantity.