LONDON, March 22 (Reuters) - Britain's antitrust regulator warned that the merger between Vodafone's UK operation and Hutchison's Three UK unit could leave consumers worse off and said the tie-up will face an in-depth probe if the pair do not provide solutions.

The $19 billion deal between the two companies was announced last year, and if it proceeds, it will reduce the number of mobile networks in Britain from four to three, meaning it was widely expected to be referred for an in-depth investigation.

The Competition and Markets Authority (CMA) on Friday gave the two companies five working days to respond with "meaningful solutions" to its concerns that the merger will result in higher prices for consumers and businesses and lower investment.

"Whilst Vodafone and Three have made a number of claims about how their deal is good for competition and investment, the CMA has not seen sufficient evidence to date to back these claims," the regulator said in its statement.

"These warrant an in-depth investigation unless Vodafone and Three can come forward with solutions." (Reporting by Paul Sandle and Sarah Young Editing by Mark Potter and Tomasz Janowski)