AIB said it has started exclusive discussions with NatWest for the acquisition of about €6 billion of Ulster Bank's performing tracker mortgages.
The bank said that any potential transaction remains subject to negotiations and agreement.
It is believed that up to 40,000 tracker mortgages could be included in the sale.
The Competition and Consumer Protection Commission (CCPC) yesterday granted clearance for AIB's proposed purchase of Ulster Bank's €4.2 billion performing commercial loan book.
The CCPC said its investigation had looked at whether the planned AIB and Ulster Bank deal would lead to a substantial lessening of competition, but concluded that it would not.
Around 280 Ulster Bank staff will transfer to AIB as part of the agreement.
But the CCPC also sounded a warning about the impact of Ulster Bank’s departure on wider competition in the banking marketplace.
In a statement today, AIB said that as intended it will start the migration of loans on a phased basis over the coming months to ensure optimised outcomes for the about 5,000 impacted customers.
AIB chief executive Colin Hunt said that receipt of CCPC approval is another important milestone in AIB's inorganic strategy and follows the successful integration of Goodbody Stockbrokers last year.
"We look forward to working with our new customers and equally our new colleagues from Ulster Bank as they move to AIB Group," he said.
"I would like to thank everyone who has worked on this landmark transaction," he added.
AIB, the largest mortgage lender here, is due to issue its first quarter trading update on May 5.
We need your consent to load this rte-player contentWe use rte-player to manage extra content that can set cookies on your device and collect data about your activity. Please review their details and accept them to load the content.Manage Preferences
Minister for Finance Paschal Donohoe said he welcomed today's news from AIB, adding that the successful conclusion of the transaction by AIB would be very positive in providing a significant number of Ulster Bank customers with certainty as to the destination of their mortgages.
"I am hopeful that these discussions will lead to a successful outcome, however, I do appreciate that the conclusion of this transaction is subject to normal due diligence, agreement of final terms, as well as obtaining appropriate approvals," the Minister said.
The Minister also said he wanted to note that the CCPC has given its clearance to AIB to complete its acquisition of Ulster Bank's corporate and commercial loan book from NatWest Group.
"Again, this is a very positive development for Ulster Bank customers whose loans will be transferring to AIB and also, for the Ulster Bank staff who will be moving to AIB as part of the transaction," he said in a statement.