Vodafone and Three have refused to rule out price rises for millions of customers as they battle to get their ￡15bn merger approved by the competition watchdog.

The two mobile phone giants left the door open to higher bills for subscribers as they responded to a probe by the Competition and Markets Authority (CMA), arguing against a wide-ranging price freeze.

However, Vodafone and Three did pledge to maintain tariffs for a portion of users on its cheapest plans.

Earlier this month, the CMA warned that mobile bills could rise by between 3pc and 6pc as a result of a merger between Three and Vodafone, which would create the UK’s biggest network and cut the number of operators from three to four.

The CMA suggested that the companies could promise “behavioural remedies” to assuage the regulator’s concerns, including allowing customers to “roll over” their existing contracts for a certain period of time.

But responding to the watchdog on Monday, Vodafone and Three said that “time-limited retail customer protections are not necessary” and that customers are “already adequately protected from price increases and service degradations”.

The parties said they disagreed with the watchdog’s main concern that the deal could see consumers lose out through higher bills or lower quality service.

Vodafone and Three argue that plans to invest ￡11bn in the combined company’s mobile network will create extra capacity that incentivises it to keep bills low, and that greater competition with rivals EE and Virgin Media O2 could lead to lower prices.

On Monday, the merging parties made limited commitments on price in response to particular concerns from the CMA.

The companies said that for two years they would freeze prices for subscribers to Three’s cut-price operator, known as Smarty, when customers are paying ￡10 a month or less.

Those on social tariffs with Vodafone’s sub-brand Voxi will also have a two-year price freeze, and the companies will not raise prices mid-contract for customers in financial distress.

Vodafone and Three also committed to competitive pricing for mobile virtual network operators, which are companies such as Sky Mobile and Lyca Mobile that rent space from mobile networks.

The companies have already pledged to have Ofcom oversee its ￡11bn network investment and to sell some capacity to Virgin Media O2 in a bid to assuage the regulator’s concerns.

“While we do not agree with the CMA’s provisional findings that prices will increase, we continue to explore how we can answer its concerns,” they said.

They called the proposed merger a “once in a generation opportunity” to boost Britain’s mobile networks.

If the deal is scuppered, they warned that “the UK would remain trapped in its current low investment, low competition equilibrium that has left it at the bottom of the class amongst European countries”.