The disclosure by the bank follows a report earlier on Wednesday which said that the SFO would open a probe into a matter already being investigated by Britain's Financial Services Authority.
Barclays disclosed the FSA investigation in July. It relates to fees paid to the Qatar Investment Authority on deals in June and November 2008, when Barclays raised 11.5 billion pounds ($18 billion).
The bank is still reeling from another investigation, after being fined 290 million pounds by British and US regulators in June for rigging Libor interest rates, sparking criticism about its culture and risk-taking and forcing its chairman and chief executive to quit.
Earlier this month, another British bank, Standard Chartered, came under the spotlight when a New York regulator accused it of dealing with Iran in violation of US sanctions.
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