(Bloomberg) -- A former banker at the center of a German trial over the so-called Cum-Ex scandal said he regrets his role in the controversial practice, pledging to help unpack the complex web of interactions that enabled the deals and cost the country billions in tax revenue.

Speaking in a courtroom in Bonn, Martin Shields, 41, laid out how he came to his role in the trading environment and how the deals were set up. Standing trial on charges that he helped orchestrate transactions that led a tax loss of more than 400 million euros ($442 million), Shields said he has a different view on the practice now than he did early on in his career.

“With the benefit of age and experience, and as a father, with very different perspectives and priorities, I look differently on the role and responsibility of financial markets, and of Cum-Ex in particular,” Shields said. “I often ask myself whether if I had my time again I would do things differently. Knowing what I now know, the answer is obvious.”

Shields is cooperating with Cologne prosecutors in a bet to avoid a lengthy prison term, helping uncover crucial elements of the transaction, as well as people and financial industry players that were involved. His testimony was closely followed by dozens of lawyers dispatched by banks and brokers probed in an investigation featuring about 400 suspects.

Key Defendant

Shields is a key defendant in the first major Cum-Ex trial in Germany, which seeks to understand how participants in the financial-services industry were able for years to get multiple tax refunds on dividend payouts, a practice the government has since sought to abolish.

Lawmakers estimate the financial engineering cost the government more than 10 billion euros in lost revenue, a shortfall the treasury is keen to recoup from those involved. The Cum-Ex scandal has caught up multiple financial institutions, including Deutsche Bank AG, Bank of New York Mellon Corp. and Societe Generale SA.

Painting a picture of a young man whose quick grasp of numbers and problem-solving landed him in world of London investment banking, Shields described how he secured his first job in finance. At the age of 21, he was armed with an engineering degree from Oxford and a keen interest in solving complex issues.

“What has always interested me was the challenge of dealing with problems and processes,” Shields told the court. “I always hoped that engineering would give me that. But I found that the routine and tempo didn’t suit my 21-year-old self.”

Speaking in English and supported by a detailed computer presentation and a laser pointer to make his argument, Shields said he wants to help the court gain a full understanding of the processes as well as the professional environment in which the trades took place.

Hard Choices

“I had to make a difficult decision: To remain silent or to testify,” Shields told the judges. “Given I stand here today I clearly decided on the latter.”

Shields said the deals required the interplay of many characters, turning the trial into a tantalizing examination of the industry at large. He rattled off a list of banks and brokers he said had participated and the various roles they took.

“Cum-Ex involved a vast network of companies, individuals and entities acting in different roles -- often in direct competition with one another -- but ultimately all were essential for the success of the trading strategy,” Shields said in his opening remarks.

Shields has already provided prosecutors with a deep dive into the world of Cum-Ex, based on what he described as more than 30 interview sessions and hundreds of hours of submissions that created almost 1,000 pages of documentation.

The trial is scheduled to last into next year.

To contact the reporter on this story: Karin Matussek in Berlin at kmatussek@bloomberg.net

To contact the editors responsible for this story: Benedikt Kammel at bkammel@bloomberg.net;Anthony Aarons at aaarons@bloomberg.net

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