The proposed merger of Vodafone and Three could lead to “higher prices” for millions of customers. Their planned £15 billion deal could now be subject to an in-depth investigation, with the UK’s competition watchdog warning it could result in a potential cost hike to users. The combined group would be Britain’s biggest mobile network with about 27 million users, and the companies said the merger would result in an additional investment of £11 billion in the UK. But the Competition and Markets Authority’s deputy chief economic adviser Julie Bon said she had “identified concerns which could lead…