ATM provider Cardtronics faces an in-depth UK Competition and Markets Authority (CMA) investigation into its multi-million pound acquisition of Canadian rival DirectCash Payments.
Cardtronics, which describes itself as the world's biggest operator of non-bank ATMs, agreed to buy DCP in a deal worth around $460 million, including debt, in October 2016.
Both firms supply free-to-use and pay-to-use machines in the UK in places like supermarkets, convenience stores and pubs.
Following its initial investigation into the deal, the CMA says that, in those local areas where there is insufficient competition from rival ATMs, the merger could lead to increased surcharge fees for customers withdrawing cash.
"Given the potential lack of suitable sites and the cost of supplying new ATMs, entry into these local areas by competitors would not be sufficiently likely to prevent an increase in fees."
The merger is now being referred for an "in-depth phase 2" investigation - unless Cardtronics offers undertakings that appease the watchdog.
The CMA has been busy in this space recently, demanding concession from Diebold and Wincor Nixdorf in relation to their merger and also securing concessions from Mastercard over its acquisition of Vocalink.