Traders in Citigroup Inc.'s (C) London office were behind some of a series of natural gas trades that prompted inquiries by U.K. regulators last month after allegations of market manipulation, U.K. daily the Guardian reports Friday.

The U.K.'s Financial Services Authority and the energy watchdog, Ofgem, launched probes into a series of six gas deals made in September after an employee at price reporting agency, ICIS Heren, raised concerns over market abuse, the paper says.

According to the Guardian, Citi traders made two out of the six gas deals under scrutiny and the bank is said to have sold gas substantially below the price of other deals made earlier in the day during the "window," the time during which price reporting agencies set the benchmark prices for the day.

Citigroup wasn't immediately available to comment.

The allegations come at a time of increased scrutiny by authorities of how energy markets are regulated and of the relationship between physical commodities markets and associated derivatives.

Full story: http://www.guardian.co.uk/business/2012/dec/06/gas-price-fixing-investigation-citigroup?INTCMP=SRCH

Write to Sarah Kent at sarah.kent@dowjones.com

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