Barclays' actions included seeking to influence the EURIBOR (European Interbank Offered Rate) submissions of other banks and reducing its own submissions due to concerns about negative media comments.
In an open letter to Treasury Select Committee chairman Andrew Tyrie MP, Mr Diamond said that only a small number of people were involved.
"This inappropriate conduct was limited to a small number of people relative to the size of Barclays trading operations, and the authorities found no evidence that anyone more senior than the immediate desk supervisors was aware of the requests by traders."
He said senior management took steps to stop the actions when it was first discovered and would be reviewing employee conduct.
He also acknowledged the firm lowered its LIBOR (London Interbank Offered Rate) submissions as it was concerned about the "integrity" of the LIBOR setting process and felt other banks' rates were too low at the time.
"Even taking account of the abnormal market conditions at the height of the financial crisis and that motivation was to protect the bank, not to influence the ultimate rate, I accept the decision to lower submissions was wrong."
Earlier this week, Andrew Tyrie MP, chairman of the Treasury Select Committee, described Barclays' actions as 'inexcusable'.
Mr Diamond said he would be happy to give evidence to the committee on the firm's actions.

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