They have been called “the men who plundered Europe”: a group of cowboy traders, seasoned tax lawyers and mathematical whizz kids who are alleged to have conspired in the heart of the City of London to siphon at least €60bn in taxpayers’ money from the state coffers of several EU countries. In Britain, the so-called “cum-ex” scandal, named after the complex derivatives juggling act employed, gained little attention amid the frenzied debate around the UK’s departure from the European Union when the fraud scheme was discovered in 2017. But in continental Europe what Le Monde has described as the “robbery of the century” has done almost as much to shape the view of Britain as Brexit itself. Dutch media has called it “organised crime in pinstripe suits” and one of the original German whistleblowers saying he now welcomes Britain’s exit from the EU in the hope it could weaken the influence of London investment banking on European financial institutions.This week, a British former investment banker involved in developing the scheme for the first time gave the public an insight into how the scheme worked and what spurred on its architects. Speaking at a regional court in Bonn, Martin Shields, one of two former bankers on trial for 34 instances of serious tax fraud between 2006 and 2011, painted a picture of a London banking scene which lured in the brightest scientists from the country’s top universities and used them to boost their profit margins – without teaching them about the moral and legal consequences of their actions in return.A cum-ex deal, from the Latin meaning with-without, is a complex set of share transactions with the purpose of getting the state to reimburse a tax that was never paid in the first place. Cum-ex transactions work by trading shares at high speed on or just before the dividend record date – the day the company checks its records to identify shareholders – and then claiming two or more refunds for capital gains tax which had in fact only been paid to the state once. Because the payout is automated, the trade can be repeated again and again.Some financial experts have likened the practice to the banking equivalent of parents claiming child benefit for multiple children when they only have one. “This was the environment at that time: a financial industry that – at least as far as I could see – was geared towards maximum profit optimisation,” the 41-year-old told a packed courtroom on Wednesday.“One tool to achieve this goal was tax optimisation: avoiding taxation as far as possible – and taking advantage of any opportunities that could be found or created. This was not the clandestine approach of a few. Rather, I saw it as the clear and openly communicated expectation of most major banks and their customers.”Hailed as a maths prodigy at school, Shields accepted a junior position at Merrill Lynch after studying engineering, economics and management at Oxford University because the trading room floor offered him a thrilling, dynamic environment. He was not alone: of 120 engineers in his year group at university, Shields added, only five went into engineering.Wearing a navy blue suit and the latest Apple Watch 5 with a white strap, Shields on Wednesday used a Powerpoint presentation to talk the court through the “cum-ex ecosystem” of labyrinthine trade chains he helped conceive and control, which prosecutors say cost the German state €450m. A translator tasked with rendering City trader jargon into German legalese was struggling to keep up.The financial rewards were breathtaking: for the five years in which Shields practised cum-ex trades through Gibraltar-based investment vehicle Ballance Capital, his personal income amounted to €12m. In 2010 Shields and his wife managed to purchase a £9.7m mansion on Chelsea’s Egerton Crescent, followed by a €6m Edwardian terrace on Shrewsbury Road, Dublin’s most expensive residential street.While Shields did not respond directly to the charges of serious tax fraud this week, he said in hindsight he had started to feel regret about devising the schemes, which hoovered up money that could have otherwise been spent on building roads, hospitals or nurseries.He told the court: “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. I would not have involved myself in the cum-ex industry.”He had made the “difficult decision” to cooperate with the investigation, which increases his chance of reducing a potential 10-year jail service. Co-accused Nick Diable (39), who worked with Shields for Germany’s fourth largest bank HypoVereinsbank (HVB), will also give a testimony in the trial, which is expected to last until next year. Shields said cum-ex trades were practised on an “industrial scale” in the first decade of the 21st century, and involved a vast network of banks, companies, brokers, lawyers and financial advisers. Even the most basic cum-ex deal involves at least 12 transactions.The banks and financial institutions he mentioned in Wednesday’s and Thursday’s court session included Clearstream AG, a 100% owned subsidiary of Deutsche Börse AG that processes the dividend compensation payments and which Shields appeared to suggest played an active part in keeping the cum-ex bonanza going after German lawmakers tried to close a loophole in 2007.