BoI notified 13,500 UK mortgage customers in February this year of its intention to increase the interest rate on their Base Rate Tracker mortgages, citing a specific clause in their contracts that allowed an increase after the guarantee period had ended.
Which? executive director Richard Lloyd said burying important changes in the small print was "wholly unfair.”
But BoI said: “This clause was clearly referenced in the pre-sale offer document provided to the customer and the customer's intermediary prior to completion.
“This change reflects the significant increase in the cost of funding these mortgages since 2008 and the need for banks to maintain greater levels of capital.”
Of the 13,500 affected customers two thirds are buy-to-let and 80% have a loan to value of 60% or less.
The bank added: “Bank of Ireland has a range of mortgages available to customers who would like to review their mortgage options.
“If a customer decides to move to an alternative mortgage provider, no early repayment charges will apply.
Find the latest alternative mortgage rates in the UK with this article.
“We are conscious of the consequences for our customers and will treat any associated difficulties with appropriate care.”
Bank of Ireland has provided a dedicated support line and encourages customers who wish to discuss the impact of the increase on them to contact the Bank in the first instance (Freephone telephone number: 0800 345 7512).
