The ad, created by the integrated agency Dinosaur, took the form of a letter from Niall Booker, the chief executive of The Co-op Bank. It follows an ad in November, which claimed that "ethical banking" has always been in the bank's DNA.
The copy in the ad said: "As many of you will be aware, 2013 was a testing year for The Co-operative Bank and as we start the New Year, I would like to say a heartfelt thank you to our customers, colleagues and investors who have demonstrated overwhelming loyalty and support throughout."
The ad ran in Sunday newspapers yesterday in a campaign planned and bought by Rocket.
A spokesman for The Co-op Bank said: "We recognise that 2013 was a testing year for the Bank and as we start the New Year it is only right that we acknowledge the overwhelming loyalty and support that has been demonstrated by our customers, colleagues and investors. The open letter is our way of reaching as many of these people as possible to extend our heartfelt thanks.
"We know that 2014 presents its own challenges, but we start the year with a strengthened capital position, management team and a business plan which is designed to return us to sustainable profitability over time. This means we can start to look forward and reshape the business around what is important to our customers."