* First criminal trial in biggest German fraud probe
* Prosecutors accuse British bankers of organising sham trades
* Banker says scheme had "industrial" scale and was no secret
By John O'Donnell
BONN, Sept 18 (Reuters) - A banker at the centre of a trading scheme German prosecutors say resulted in hundreds of millions of euros of illegitimate tax rebates told a court the scheme had taken on an "industrial scale" involving a network of banks and other institutions.
Martin Shields, a former investment banker, said that a web of banks, investors and brokers had organised the circular trades, making multiple tax reclaims and sharing the profit.
"I am not before you to deny my involvement, but to explain it," Shields told the court in Bonn on Wednesday.
The trial is the first in a wider investigation aimed at recovering billions from banks which prosecutors say profited from such schemes. Germany estimates the schemes cost it more than 5 billion euros ($5.5 billion) in total.
Prosecutors allege that players in the so-called cum-ex scheme misled the state into thinking a stock had multiple owners who were each owed a dividend and a tax credit.
Shields' testimony is a critical building block in pursuing others potentially involved. The case, Germany's biggest post-war fraud investigation, is being closely followed in London and Frankfurt, where much of the trading was organised, according to bankers and court documents.
German state prosecutor Anne Brorhilker this month outlined criminal charges against Shields and fellow British banker Nicholas Diable, who she said organised a network of traders and lenders to make double tax reclaims with sham share trades.
Brorhilker said the two men, who face a possible jail term and a state order to repay any profits they made, had been involved in organising bogus share trades in companies including carmaker BMW and airline Lufthansa.
The alleged scheme involved trading shares rapidly around a syndicate of banks, investors and hedge funds to give the impression of numerous owners, each entitled to a tax rebate.
Shields, a 41-year-old engineering graduate, said he regretted his involvement in the trading scheme but that the practice was common knowledge in the industry and that he had no reason to believe at the time that it was legally questionable.
"This was not the clandestine approach of a few," he said, describing it as the "clear and openly communicated expectation of most banks and their customers".
Diable is expected to address the court on Thursday. His lawyer did not respond to a request for comment.
Brorhilker has estimated that the alleged scheme she is prosecuting cost Germany some 450 million euros in lost taxes.
She said the two men had started it when working at German bank HVB, before moving to a small investment firm, tapping a wide network of contacts to arrange the trades, sometimes splitting the profit made with banks.
Shields named banks and financial institutions he said were involved in the scheme, including Warburg, Deutsche Bank and Deutsche Boerse's trade settlements arm Clearstream.
Last month, prosecutors raided the offices of Deutsche Boerse as part of the wider probe.
Warburg has been included in the German trial and may have to repay any profit made from the schemes. A spokesman for Warburg has said that it had not received such multiple tax rebates and had never set out to do so.
HVB, which has blamed its involvement on wrongdoing by individuals, declined further comment. A spokesman for Deutsche Boerse said it was fully cooperating with the authorities.
A Deutsche Bank spokesman said it had been involved with clients' cum-ex deals but that it now takes a critical view of these transactions and is cooperating with the authorities.
Germany changed and clarified the law in 2007, 2009 and 2012 to prevent cum-ex trades, but the case has fuelled mistrust of banks among ordinary Germans.
The discovery of the trades in Germany and Denmark, where tax authorities say they lost $2 billion in rebates, prompted investigations by other countries including Austria and Belgium, which have found they were also affected, albeit on a far smaller scale.
$1 = 0.9039 euros Reporting By John O'Donnell; Editing by Nick Tattersall