The Co-operative Bank is to be the subject of an enforcement investigation by the Bank of England's Prudential Regulation Authority (PRA).
The regulator said the probe would "consider the role of former senior managers" at the lender, which underwent a rescue last year after a £1.5 billion black hole was discovered in its finances.
The shortfall was widely attributed to problems arising from the takeover of the Britannia building society and the failed bid to acquire more than 600 branches from Lloyds.
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Questions about the management of the bank were also raised following drug revelations surrounding former bank chairman Paul Flowers.