Cabinet Secretary Sir Jeremy Heywood was criticised by MPs for his investigation into former chief whip Andrew Mitchell's "plebgate" row with police officers at Downing Street.
Sir Jeremy was not the "appropriate figure" to carry out such an inquiry and suggested that his role as an "impartial investigator" was compromised, The Commons Public Administration Committee said.
Mr Mitchell was forced to resign following claims that he swore at the officers and called them "plebs" after they refused to allow him to cycle through the main gates at Downing Street.
However, evidence has emerged casting doubt on the police account of events - which Mr Mitchell has always strongly disputed, insisting he did not use the word "pleb" and only swore once under his breath.
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