Vodafone’s agreement to merge its U.K. business with rival operator Three raises competition concerns, the U.K.’s antitrust authority said, creating uncertainty on a deal poised to shake up the country’s telecommunications market.

Vodafone's merger with Three UK could win regulatory approval if they can guarantee the $19 billion deal will be good for consumers, network quality and competitors like Sky Mobile, Britain's competition watchdog signalled on Friday.

The UK warned a £15 billion ($19.7 billion) merger between Vodafone Group Plc and Three could undermine investment and lead to potential price increases, while still leaving the deal a path to completion.