Britain's competition watchdog has voiced concerns over Mastercard's £700 million acquisition of VocaLink, which runs Bacs, Faster Payments and Link cashpoint services for banks.
The Competition and Markets Authority (CMA) believes the deal gives rise to competition concerns in the Link ATM network.
It said the merger would "reduce the number of bidders and limit the ability of the Link scheme to obtain good value" when tendering for an infrastructure provider.
The companies can avoid the merger being referred for an in-depth CMA investigation if they offer a remedy to address the concerns, the watchdog said.
Andrea Coscelli, acting chief executive of the CMA, said: "The Link ATM network provides an essential service for millions of customers. It's important that Link has a good choice of providers when it comes to supplying the necessary infrastructure so it can take advantage of the opening up of payment systems to competition.
"These concerns warrant a closer investigation in the event that Mastercard cannot address them at this stage."
A group of 18 banks - including HSBC, Royal Bank of Scotland, Lloyds Banking Group and Barclays - owns VocaLink, which processes more than 90% of salaries, over 70% of household bills and almost all state benefits in the UK.