The watchdog stepped in after a deposit rate advertised by the bank allegedly failed to state that the account was liable for Dirt tax. The ad also failed to remind customers that the value of investment-based accounts could go down as well as up.

The regulatory move is embarrassing for Bank of Ireland, which tried to steal a march on the competition by rolling out its SSIA followon products early. These include a market-leading deposit rate of 3.75% and a promise to continue an SSIAstyle savings top-up for customers who choose an investment account.

A spokesman for the regulator said: "We monitor advertisements of banks and other financial institutions on an ongoing basis. If we come across something that's potentially misleading for consumers, we raise it with the institution." A Bank of Ireland spokeswoman said it was cooperating with the regulator's investigation. "As we understand it, we're fully compliant with the code on advertising. But we can't comment on any current queries from the regulator while they are still ongoing."