Its not known how many of the 32 thousand customers whose loans were transferred are affected.
Some mortgage customers of AIB have been caught up in an undercharging mistake.
Its reported those with tracker loans transferred to AIB when Ulster Bank left the Irish market, are being told their loans were miscalculated.
The Irish Independent reports letters have been sent to customers this week informing them they'll have to pay more on their repayments from October.
Its not known how many of the 32 thousand customers whose loans were transferred are affected.
In some cases they're being told they need to pay an extra €600 a month.
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