The Progressive Building Society turned down a 2011 offer from former Co-op boss Paul Flowers to buy the business, the Belfast Telegraph can reveal.
Mr Flowers, who was a trained Methodist minister, visited the Progressive's Belfast headquarters in 2011 to put his proposal forward.
But the mutual, which is Northern Ireland's only home-grown and locally-owned lender, turned down the offer.
Two years later, Mr Flowers became known as the Crystal Methodist after he was exposed for drug-taking, while the bank suffered losses of over £1.5bn.
It has since closed its only Northern Ireland branch, which was located in Belfast.
In today's Big Interview, Progressive chief executive Darina Armstrong says the building society was glad it turned down the offer from the Co-op, but that she was impressed at the research non-executive chairman Mr Flowers had done into the business.
She said: "We passionately believe that remaining independent is for the better interests of all our members."
The company posted a £14m pre-tax profit in 2015 as mortgage lending soared by a fifth in one year.
Mrs Armstrong today tells Business Telegraph that the society is continuing to see healthy mortgage lending during 2016.
Progressive, which Mrs Armstrong joined as an accountant in 1992, has just over 89,000 savings members and more than 25,000 mortgage members.
The society prides itself on prudence and caution in its lending policies and stayed in the black during the downturn.
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