Philipp Hildebrand, who resigned two days ago as chairman of the Swiss National Bank in the midst of an alleged insider trading storm, is facing up to conflicting accounts of events in a tranche of his private emails released by the bank.

The scandal concerns Hildebrand's alleged involvement in his wife's purchase of $500,000 US currency, only weeks before the Swiss National Bank capped the Swiss Franc. His wife later sold the currency back at a higher rate, making a substantial profit. If Hildebrand played any part in the actions, he would have broken the law.

Hildebrand has said he never encouraged or authorised such purchases, and only quit because he was unable to unequivocally prove his innocence. But his private financial adviser, Felix Scheuber, wrote in an email last year that Hildebrand had indicated clear approval for dollar transactions.

The emergence of the emails are reported to have played a key part in Hildebrand's departure, though the bank has not confirmed this officially.

In the emails, sent last August, Hildebrand wrote to his private financial adviser Felix Scheuber that he was "surprised" about the large US currency purchase by his wife, adding that the adviser was "not authorised" to execute any such transactions without his approval. He said that they had "never discussed" any such purchase, even though she had written that they both wanted to increase their "dollar exposure".

Scheuber replied, disputing Mr Hildebrand's comments. He added: "I also remember you saying in our yesterday's conversation [sic] that if [your wife] Kashya wants to increase the USD exposure then it is fine with you."

The bank said today that it was stepping up its governance arrangements, and looking for a new chairman.