The UK government's decision to retrospectively order Barclays to close down two tax loopholes earlier this year was "completely unwarranted" and caused unnecessary damage to the bank's reputation, chief executive Bob Diamond said in a letter to the Treasury Select Committee that was released Monday.

In the letter, dated May 15, Diamond wrote that the tax situation arose after the bank voluntarily told the tax department, Her Majesty's Revenue & Customs, that it had bought back some of its own debt in a "tax efficient manner".