April 4 (Reuters) - Britain's competition regulator said on Thursday it would need to open an in-depth investigation into the merger between Vodafone's UK operation and Hutchison's Three UK after the parties did not offer remedies to ease the regulator's competition concerns.

The $19 billion tie-up, which was announced last year, will reduce the number of networks from four to three, challenging the long-held tenet that four help keep prices low.

(Reporting by Yadarisa Shabong in Bengaluru; Editing by Janane Venkatraman )