By Scott DiSavino and Jonathan Leff

WASHINGTON/NEW YORK (Reuters) - U.S. regulators threatened to fine Barclays roughly $470 million to settle allegations that the bank and four traders manipulated California electricity markets, reviving the specter of a sector-wide crackdown on energy trading.

It could possibly be the biggest penalty ever levied by the Federal Energy Regulatory Commission (FERC), and potentially exceeds the fine Barclays paid over the Libor bid-rigging scandal that cost Chief Executive Robert Diamond his job.

The bank has 30 days to show why it should not be penalized for an alleged scheme of manipulating physical electricity prices at a loss in order to make profits in related positions in the swaps market, a strategy known as a "loss-leader".

British bank Barclays said it would fight the agency, likely setting up a landmark legal battle that could set a precedent over whether the once-common trading ploy in commodity markets is illegal or simply ill-advised.

It will have huge implications across the market, as the FERC -- which won expanded powers to tackle manipulation in 2005 after the California power trading scandal and related Enron meltdown -- pursues similar investigations against companies including BP and Deutsche Bank.

The FERC also said four of the company's power traders -- Daniel Brin, Scott Connelly, Karen Levine, and Ryan Smith -- have 30 days to show why they should not be assessed a total of $18 million in civil penalties.

It said their activity accounted for nearly a quarter of all trading in the next-day power market during the period, accruing gains of an estimated $34.9 million. Bank documents showed how the traders bragged about how they would "crap on" certain markets to profit in other ones, the order shows.

Barclays "strongly disagreed" with the order, which it said was "by nature a one-sided document, and does not reflect a balanced and full description of the facts."

"We believe that our trading was legitimate and in compliance with applicable law," Barclays spokesman Mark Lane said in an email. "We have cooperated fully with the FERC investigation, which relates to trading activity that occurred several years ago. We intend to vigorously defend this matter."

The four traders left Barclays over the past five years for reasons unrelated to the investigation, according to a source familiar with the matter. The bank closed its Portland office in 2011 and effectively quit the Western power market this year.

It is the latest blow for Barclays, which has fired staff, clawed back pay and taken other disciplinary action after being fined $450 million by U.S. and British regulators over Libor.

New CEO Antony Jenkins, who took over at the end of July, is in the middle of a review to change the bank's culture and lift profitability. The changes are due to be unveiled in February.

Earlier on Wednesday, Barclays announced that the U.S. Department of Justice and the Securities and Exchange Commission were investigating whether it was complying with U.S. laws in its ties with third parties who help it win or retain business.

The FERC order is tantamount to an indictment, suggesting the issue may go to court after settlement talks were unsuccessful, said Craig Pirrong, a University of Houston professor and expert in energy trade regulation.

EMBOLDENED FERC

The order threatens to stir up memories of the California power crisis, but the allegations involve a far more subtle trading strategy that industry veterans say had been common in global markets: using loss-making trades in benchmark physical commodity markets in order to profit from derivatives positions.

The "loss leader" gambit had until recently been viewed as outside the bounds of regulators, whose purview typically covered either physical markets or derivatives, but rarely both. Although the practice has diminished greatly in recent years as regulators get tougher, many veterans fear that old deals could come back to haunt them.

As well as the return of $34.9 million gains plus interest, the commission is also seeking a $435 million civil penalty.

"Scary stuff," said one senior executive at a trading firm. "Which I guess is the point."

The FERC first flexed its muscles earlier this year with a record $245 million settlement with power company Constellation Energy over similar allegations. Other agencies pursued larger fines against power merchants after the California debacle.

"FERC is getting tougher," said Pirrong. He cautioned that there are "factual and conceptual challenges" in proving manipulation in court, but that isn't stopping the agency.

"It is going to town on this theory of manipulation, and I would wager that any firm that traded both physical and financial power is at risk of a similar FERC action," he said.

WARNING IGNORED

The FERC Office of Enforcement staff alleged Barclays engaged in a coordinated scheme to manipulate trading at four electricity trading points in the Western United States.

The order alleges that Connelly, hired in May 2006 to start a North American power market desk, hired a team of traders who ultimately came to dominate the market, making up 24 percent of all next-day fixed-price trading on the IntercontinentalExchange trading platform during the months in question.

The bank's "total market concentration" was as much as 58 percent and no less than 10 percent in any given month, it says.

It then engaged in "loss-generating trading of next-day fixed-price physical electricity" at a number of key electricity hubs in order to pay off positions in the swaps market, where contracts are settled based on physical market prices.

The traders were aware that the trading was "likely unlawful", and ignored the warning of the head of Americas commodity trading Joe Gold, who "made clear the practice was unacceptable", according to the FERC order.

Experts say one of the biggest challenges in winning any manipulation cases is demonstrating that traders intentionally engaged in a losing trade to make bigger profits.

But, just as recorded telephone conversations between California power traders about driving up power prices for "grandma Millie" proved damning a decade ago, the agency used instant messages and emails to bolster its case.

Smith, for instance, described how he manipulated the Palo Verde market, according to the FERC order, and the "NP light" -- or off-peak power ('light') in the North Path 15 (NP) market in northern California in an attempt to "drive the SP light lower."

Levine asked colleagues to "keep the PV index up and the SP daily index down" while she was on vacation.

Mike Masters, co-founder of Better Markets and a frequent advocate for stronger rules, said it could be the "tip of the iceberg" as regulators probe more deeply into commodities.

"It's a very significant fine and not just because of the dollar amount. It also highlights how banks and swap dealers were combining financial and physical positions in a predatory way to manipulate commodity markets," he said.

"If it's happened in power markets you can be sure it was also going on in crude oil and other markets like refined oil products," Masters said.

(Reporting by Scott DiSavino, David Sheppard, Jonathan Leff and Karey Wutkowski; Editing by Gary Hill, Andrew Hay, Claudia Parsons and Paul Tait)

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