LONDON, March 22 (Reuters) - Britain's antitrust regulator warned that the merger between Vodafone's UK operation and Hutchison's Three UK could leave consumers worse off and told the pair to provide it with solutions or face an in-depth probe.

"Both Vodafone UK and Three UK have five working days to respond with meaningful solutions to the CMA (Competition and Markets Authority), otherwise the deal will be referred to a more in-depth Phase 2 investigation," the CMA said in a statement on Friday. (Reporting by Paul Sandle and Sarah Young Editing by Mark Potter)