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FRANKFURT (Reuters) - Hamburg-based bank M.M. Warburg & Co said on Thursday it had filed a lawsuit against Deutsche Bank DBKGn.DE seeking 46 million euros ($53.04 million) that the bank was ordered to pay in capital gains tax.

Warburg said the lawsuit was filed in December in a Frankfurt regional court.

Warburg said Deutsche Bank had failed to withhold taxes of 46 million euros for a series of share trades that took place as part of Deutsche’s role as a custodian bank for the years 2010 and 2011.

Hamburg tax authorities had told Warburg to repay those funds, and this prompted the legal action, the bank said. Warburg also denied any wrongdoing.

A Deutsche Bank spokesman said the bank had not seen the court filing but also said that Warburg’s allegations as reported in the German media were “without merit” and that the bank “strongly rejected” the accusations.

Warburg’s lawsuit relates to a share-trading scheme - known as “cum-ex” - that has become the subject of Germany’s biggest post-war fraud investigation. Authorities say the scheme cost taxpayers billions of euros.

Public prosecutors in Cologne and elsewhere in Germany have said the players in the so-called cum-ex scheme misled the German government into thinking a stock had multiple owners on its dividend payday who were each owed a dividend and a dividend tax credit. The prosecutors have also said the scheme was illegal and misled the German government into paying tax refunds.

Deutsche Bank has said it is fully cooperating with investigators regarding the scheme.