AIB has been accused of refusing to speed up mortgage applications to allow new buyers meet a deadline to get generous tax reliefs.
The relief is worth up to €18,000 for a couple who complete a contract and draw down their mortgage before December 31.
But brokers said they have been forced to encourage clients to go to lenders other than AIB, after the bank refused to process mortgage approvals in time for new buyers to meet the deadline.
Carol Tallon of Buyers Broker said AIB was sticking to its normal deadlines, despite a rush of buyers eager to complete a purchase.
Other banks are fast-tracking mortgage applications, with Ulster Bank putting in place a 24-hour turnaround for new buyers, she said.
Finance Minister Michael Noonan has repeatedly said the mortgage tax relief will not apply for those who do not draw down a mortgage and complete contracts before December 31.
For a new-buyer couple purchasing a €200,000 house, the tax relief could be worth more than €2,000 next year alone.
The scheme means first-time buyers receive mortgage interest relief at 25pc while anyone who has previously owned a home receives relief at 15pc.
The relief is paid by the Revenue Commissioners to the mortgage lender for five years, and effectively means borrowers make smaller monthly repayments.
The scheme means a saving of €18,000 over seven years for a couple taking out a €300,000 mortgage at a 4.25pc interest rate, if they buy a house before the end of this year.
The Government has described the tax relief as the "offer of a lifetime".
A spokesperson for AIB denied that the bank was delaying the mortgage approval process for new buyers who need to complete a sale in the next seven weeks.
"Anyone who feels that they may miss the deadline to qualify for mortgage tax relief because of delays in getting mortgage approval should flag it and we will resolve to process the application," the spokesperson said.
But Ms Tallon, who acts on behalf of those buying a home, said: "AIB won't escalate files, or speed up the mortgage approval process, even though the volume of mortgages they are processing is really low."
She has a number of first-time buyer clients who are rushing to complete a purchase but the AIB situation has meant that Ms Tallon has advised them to seek mortgage approval from Ulster Bank instead.
There is a race against time to meet the deadline as conveyancing by solicitors can take seven weeks, she added.
The broker said it should be possible for anyone who finds a house they can afford, to have the legal work and the mortgage approved before the end of the year.
But some buyers, she said, were under the mistaken impression that they only have to choose a property before the end of the year to qualify for the mortgage tax relief.