COPENHAGEN — When a whistle-blower alleged that employees of a branch of the Danish lender Danske Bank had been knowingly working with customers who had broken the law, it was not the first time that questions had been raised about that particular office. It would not be the last.

Danske Bank said on Wednesday that its headquarters and its Estonian branch failed for years to prevent suspected money laundering involving thousands of customers. The lender said it was unable to estimate the total amount of the suspicious transactions, but its nonresident operation in the Baltic nation improbably had total flows of 200 billion euros, or $234 billion — nearly equivalent to the size of the Danish economy. The chief executive, who had previously headed the bank’s international operations, quickly said he would resign.

In an 87-page report commissioned and paid for by Danske Bank, a Danish law firm, Bruun & Hjejle, found that misconduct took place at the lender’s Estonian branch from 2007 to 2015, involving “objectionable” omissions, inaction and faulty processes at all levels of the bank. That meant the lender “was not sufficiently effective in preventing the branch in Estonia from being used for money laundering.”

“None of us can grasp that this had been going on,” Ole Andersen, Danske Bank’s chairman, said in a news conference on Wednesday.