The Competition and Markets Authority said it could deepen its probe, after launching in January an investigation into the deal, should the pair fail to address regulator concerns.

The proposed tie-up, announced in June last year, is aimed at creating Britain’s biggest mobile operator with 27 million customers and to accelerate rollout of faster 5G connectivity.

In a statement Friday, the CMA said it is “concerned the deal… could lead to mobile customers facing higher prices and reduced quality”.

Vodafone and Three said in response that they would review the CMA’s concerns.

The planned transaction sees British giant Vodafone taking 51% of the combined group and CK Hutchison the rest.

The pair have a target value of £16.5 billion (US$21 billion) for the new group.

The merger, if approved, will vault the combined operations above the country’s two largest mobile operators BT EE and Virgin Media O2 in terms of customer numbers.

Expansion of 5G across the UK has been hampered by Britain’s ban on Chinese giant Huawei, a major supplier of equipment for mobile telephone networks.