Ulster Bank said the first customers who received their six-month notice in April will begin to see their current and deposit accounts frozen on or after Friday 11 November, with the accounts to close 30 days later.
The bank said the process will start with customers who it believes have low reliance on these accounts or may have accounts elsewhere.
In line with its commitment to withdraw from the Irish market in an orderly manner, Ulster Bank said it is starting the process in a "careful and controlled way".
Over 70% of Ulster Bank personal current account customers who received their first formal notification in April and May, have either closed, materially wound down the level of activity in their current account or left it inactive.
This trend is increasing every day, the bank said.
Ulster Bank also said today it will reach out again to some customers to offer further support, adding that these higher reliance accounts will not be frozen at this time.
These include personal or commercial current account customers with six or more transactions in the last 30 days, as well as customers who have received a social protection payment in the last 30 days.
They also include personal current or deposit account customers in receipt of an inbound payment of €125 or more, in the last 30 days as this may be their wages and commercial current accounts with a reliance on an overdraft and an account turnover of more than €1,000 in the last 30 days.
Ulster Bank said it is encouraging customers who have not yet taken action to begin the process to choose a new provider, move their transactions and close their current and deposit accounts within their notice period.
"This precautionary, careful and controlled approach of freezing an account 30 days before closure means that where a customer needs more support, which may or may not mean more time, we can and are keen to provide that - they just need to let us know what they need," the lender said.
"This phased approach to overall closure of accounts is managed to maintain an orderly process for customers, the industry and other key stakeholders," it added.
Ulster Bank also confirmed that 25 branches will close on 6 or 13 January and reopen shortly thereafter as a Permanent TSB branch.
It said that if a customer's local branch is becoming a Permanent TSB branch, and if their mortgage is transferring to Permanent TSB, their current and deposit accounts are not transferring, and so action must be taken to choose a new provider.
Ulster Bank chief executive Jane Howard said the bank's focus remains on supporting customers to move to another financial institution.
"We are starting our freezing and closure process very carefully, with customers who we believe have low reliance on these accounts or may have accounts elsewhere," she said.
"At this point, we are also excluding and reaching out again to personal customers who have received a social protection payment in the last 30 days or who have received an inbound payment of €125 or more in the last 30 days.
"This careful and controlled approach means that where a customer is still reliant on the current account, we can and will support them to move to a new banking provider.
"This will also serve to maintain an orderly process for customers the industry and other key stakeholders. While we will continue to reach out to customers who are still using their account, unless a customer engages with us to change the date, their account will enter the freezing and closure process on or after their original date."
About 900,000 retail accounts are expected to move over the next year by customers of both Ulster Bank and KBC Bank Ireland as the two banks leave the Irish market.
The Financial Services Union (FSU) has said the reports that Ulster Bank intend to press ahead with freezing thousands of customer accounts this week is "totally unacceptable" and should be blocked by the Central Bank.
"It is unconscionable to think that a customer who is still fully or partly reliant on their Ulster Bank account will be forcibly rebanked by having their account frozen and as a result will have no access to their own money," said John O'Connell, General Secretary of the FSU.
"This action raises serious concerns for the right of citizens to have a bank account and for the health and safety of staff as irate customers look to regain access to their money," he added.
In a statement, Mr O'Connell called for a clear statement from Ulster Bank and the regulator to reassure customers that no person will lose access to their own money or will go unbanked.
"Alarming people with the threat of freezing their account is not acceptable," he said.
"It is obvious from the recent CCPC survey that there are real problems in the system that is preventing people from closing their Ulster Bank account and switching to new providers. The survey cited 60% of customers experiencing problems. We need to have full transparency on the barriers facing people trying to close and open accounts.
"You cannot close a 200-year-old institution with over a million customers in six months."
