Vodafone and Three have denied their proposed merger will lead to higher prices for mobile phone customers.
In response to questions from MPs, representatives from the firms said there were no price hikes built into the merger plans.
Three general counsel Stephen Lerner told the Business and Trade Committee: “It's not part of the transaction rationale, and we are not planning any increases in prices."
Vodafone announced the long-rumoured £7 billion merger of its UK business with CK Hutchison-owned Three in June and has been battling to defend the deal against competition worries ever since.
The deal will create the UK’s largest network with around 27m customers.
Lerner said he was "confident" UK regulators would approve the merger, which has been sweetened by the promise of £6bn of investment in its first five years
George Stevenson, an investigator for the Unite union, said it believed the deal would result in bills rising by £300 a year.
“Combining Vodafone and Three would be bad for consumers, 2 he told the committee.
“If we have this merger take place, we're going to see price rises, we're going to see profits go up."
Andrea Dona, network and development director of Vodafone UK, said the combined firm planned to use 5G to deliver fibre broadband equivalent internet access to 82% of UK households.
Dona also suggested its presence in the "mobile virtual network operator" (MVNO) marketplace might even see lower prices across the market as it would provide an alternative to either EE or Virgin Media O2.
"We've done a study that shows that can bring up to a £15 reduction in the bill a month," Dona said.
Shares in Vodafone rose by 0.5% to 77.9p.