The Commons Public Administration 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.
"Given time, attention and with his relevant experience, Sir Alex Allan (a former chairman of the Joint Intelligence Committee) might well have uncovered the truth."
The Commons Public Administration Committee listed a number of errors in Cabinet Secretary Sir Jeremy Heywood's inquiry into former chief whip Andrew Mitchell's "plebgate" row.
- The committee said it was "surprising" that Sir Jeremy had not been aware of a note of a conversation that the No 10 head of security and the Prime Minister's principal private secretary had with the officer concerned shortly after the incident.
- They said it was "equally surprising" that he did not establish whether a reported leak of the police log was actually true.
- Sir Jeremy should have advised David Cameron to refer the matter to the relevant police authorities so they could resolve any "discrepancies and inconsistencies", they added.
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.
Cabinet Permanent Secretary Sir Jeremy Heywood will give evidence this morning to the Public Administration Select Committee regarding the events leading up to the resignation of Andrew Mitchell as Chief Whip.
David Cameron asked Sir Jeremy to investigate the claims when they were initially reported in September 2012.
It is thought he watched the CCTV footage of Mitchell's alleged confrontation with police officers which appeared to contradict the version of events detailed in a police log.