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How Barclays turned a $10 bln profit into a tax loss

* Barclays gained unusual tax advantage from Luxembourg law

* Campaigners want law changed, cite taxpayer support for banks

* Bank says it did not use Luxembourg to get tax cut

* Says benefit was unexpected and unplanned

By Tom Bergin

LONDON, April 29 (Reuters) - When Barclays Plc sold a fund management business to U.S. financial group Blackrock Inc. in 2009, the larger-than-expected $15.2 billion price tag was not the only good news for the British bank's investors.

The way Barclays structured the sale -- by booking part of the proceeds in Luxembourg -- allowed it to do something not possible under most tax systems: generate a tax loss from a tax-exempt transaction, a Reuters analysis of previously unreported company filings and statements shows.

The move has helped Barclays to earn billions of dollars almost tax free.

The entirely legal deal is the latest example of the ways in which some companies are able to benefit from tax regimes that regulators around the world are trying to crack down on so they can raise more tax revenue at home.

The small European state of Luxembourg is among those coming under scrutiny for its tax regime that local authorities and lawyers say is a legitimate way to attract business.

Barclays' tax loss was made possible because it sold its Barclays Global Investors (BGI) business tax free in Britain, but had part of the sale proceeds -- $9 billion in Blackrock shares - paid to a subsidiary in Luxembourg.

That way, Barclays was able to offset the risk of the shares losing value, something not normally possible in a tax-free deal. A rise would have netted Barclays profits. When instead the shares fell, Barclays used the loss to claim a tax deduction in Luxembourg that was not available in the UK.

Barclays' subsidiary in Luxembourg, one of Europe's smallest states with just half a million people, lost $2.6 billion when the Blackrock shares fell, but has earned almost double the amount virtually tax free since 2012, partly by offsetting some of the Blackrock loss.

Barclays spokeswoman Candice MacDonald said the structure of the BGI sale was not aimed at securing a tax reduction but intended to secure a simpler and more certain tax treatment and avoid volatility in the bank's regulatory capital. Blackrock declined comment.

Tax advisers say there is nothing wrong with companies organising their affairs to take advantage of generous tax treatments offered by different countries.

"It would be very odd to criticise that or say it's inappropriate," said Neal Todd, tax partner at Berwin Leighton Paisner. "If governments aren't happy with the law, they should change it."