Cabinet Secretary Sir Jeremy Heywood has been sharply criticised by MPs for his botched investigation into former chief whip Andrew Mitchell's 'plebgate' row with Downing Street police officers.
The Commons Public Administration Committee said Sir Jeremy was not the "appropriate figure" to carry out such an inquiry and suggested that his role as an "impartial investigator" was compromised.
Mr Mitchell was forced to resign following claims that he swore repeatedly at the officers and called them "plebs" after they refused to allow him to cycle through the Downing Street main gates.
However, evidence has since 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.
An email supposedly sent by a member of the public who witnessed the encounter backing the police version was found to have come from a serving police officer who was not even there.
CCTV footage from the scene also appeared to undermine police claims that the row was overheard by bystanders outside the Downing Street gates who were shocked at Mr Mitchell's behaviour.
Scotland Yard has now mounted an investigation into what happened.
When he carried out his inquiry, Sir Jeremy failed to reach a conclusion on who was telling the truth, putting the discrepancies between the two versions down to a "genuine difference of view".
However, the committee listed a number of errors in his inquiry.
It was "surprising", the committee said, that Sir Jeremy had not been aware of a note of a conversation that the Number 10 head of security and the Prime Minister's principal private secretary had with the officer concerned shortly after the incident.
It was "equally surprising", it said, that he did not seek to establish whether a reported leak of the police log of the incident in The Daily Telegraph was actually true.
Given the doubts raised by the CCTV footage and the refusal of the author of the email to co-operate with his inquiry, the committee suggested Sir Jeremy should have advised David Cameron to refer the matter to the relevant police authorities so they could resolve any "discrepancies and inconsistencies".
Instead, Sir Jeremy took the view that it was not his "job" to report the inaccuracies and unanswered questions to the police.
Overall, the committee said it was "regrettable" that the matter was not dealt with by the Prime Minister's adviser on ministerial interests Sir Alex Allan - a former chairman of the Joint Intelligence Committee - who was the right person to handle such inquiries.
The committee chairman, Bernard Jenkin, said: "This underlines the all-too-obvious truth that investigations into ministerial misconduct are not an appropriate role for the Cabinet Secretary to undertake."