The Central Bank has told the banks in Ireland that they need to do more to ensure that Ulster Bank and KBC Bank Ireland customers are catered for when the two banks leave the country.
About 900,000 retail accounts are expected to move over the next year by customers of the two banks.
The Central Bank has written to the CEOs of the banks to ensure that customers are protected amid what it called the "profound structural changes" taking place in the retail banking landscape.
It said the core issues include notification periods, continuity of service for customers and assistance for vulnerable customers.
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It said that since the announcements by the two departing banks in February and April last year, it has engaged intensively with the five banks to assess and manage the implications of the transactions.
As the process of closing bank accounts now starts, the regulator said it is imperative that departing and remaining banks demonstrate that they have sufficient plans, preparations and resources in place to deliver on expectations.
"We are assertively supervising the banks to ensure they prioritise the interests of customers and prospective customers throughout this unprecedented volume of account migration," the Central Bank's Director General, Financial Conduct Derville Rowland said.
"I acknowledge the unprecedented scale involved, and also acknowledge that staff within the banks are working extremely hard in challenging circumstances to provide customers with the services they require.
"We are keenly aware of the impact on both staff and customers in that regard.
"But while recognising the challenge an exercise of this scale represents, it is also clear that, in terms of the banks' overall plans, more needs to be done."
Ms Rowland said the Central Bank will convene a roundtable of CEOs focused on the retail banks' plans to ensure consumers best interests are protected.
The Central Bank will also repeat its review of call wait times on customer support phone lines in the main retail banks in the coming weeks.
The findings of the previous review resulted in action plans to ensure the level of customer service provided on support phone lines is sufficient to manage both normal call volumes and to deal with surge events.
The Central Bank also said that banks need to engage with other service providers, including direct debit originators (DDOs), to ensure a smooth and timely switch of a customer's account.
As a number of the top 20 DDOs are regulated financial service providers, including insurance companies and payment institutions, the Central Bank has written to the CEO of each of these regulated firms to reinforce their duty to take action to ensure this exercise is completed efficiently.
Labour's Finance spokesperson Ged Nash said the move by the Central Bank today was an "important intervention", and that the Central Bank has said all along it would take a customer focused approach to this unprecedented challenge.
"I was gravely concerned three weeks ago, when I was told that both the exiting banks and receiving banks are not ready to deal with the scale of the challenge to the satisfaction of the Central Bank," he said.
"It's interesting and important that they have issued this invitation today, they need to make it clear to the CEOs of the bank what they expect of them.
"Sometimes good customer service is not a phrase we associate with banks of this country," he said.
Mr Nash said that this was because banks had closed so many branches and let so many staff go - and that he has some sympathies for those working in banks as there are going to be a "tsunami of changes" coming and he "remains to be convinced that the banks are prepared for this".
Suggesting that the Government delays the exit of Ulster Bank and KBC, he said, "they need to use every tool at their disposal to ensure that this is as seamless a transfer as possible".