The mortgage lender has come under heavy criticism for the move, first announced in February. Consumer group Which? has created an online tool designed to bombard the Bank of Ireland with complaints while landlords have launched a class action contesting the legality of the rate rise.

However, Financial Conduct Authority chief Martin Wheatley told MPs the regulator would not act as the terms were not unfair.

Two-thirds of those affected are buy-to-let borrowers and 80% have a loan-to-value of 60% or less.

Speaking shortly after the rate rise was announced, affected mortgage broker Caroline Grierson told Your Money‘s sister title Mortgage Solutions she felt the move was underhand.

“I feel cheated by the fact they’ve put it up because in my mind a tracker rate tracks at a certain level above base, and that’s it. I’ve thought that for the whole of the last ten years,” she said.

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In a response to the criticism, the bank said the increase was permitted by a specific clause in the mortgage contracts.

It stated: “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. This rate increase does not affect Post Office customers.”