PM and Cabinet Secretary pilloried for 'Plebgate' farce

Sir Jeremy Heywood: The Cabinet Secretary was in charge of the Andrew Mitchell investigation (PA)

David Cameron faces fresh embarrassment over the "Plebgate" affair after MPs concluded the wrong government official was put in charge of investigating allegations that Andrew Mitchell swore at police.

A Commons committee tomorrow delivers a damning assessment of the inquiry conducted by Britain's most senior civil servant, Sir Jeremy Heywood, into the conflicting accounts of the altercation.

Mr Mitchell was forced to resign as Chief Whip after spending more than a month denying claims – contained in police records – that he described officers who stopped him from wheeling his bicycle out of Downing Street's main gates as "f***ing plebs".

Doubts over the police version have since been raised by CCTV footage of the incident and after it emerged that an email from an eyewitness was allegedly sent by a police officer.

The implication of the Public Administration Select Committee report is that Mr Mitchell could have won his battle to keep his job if Sir Jeremy, the Cabinet Secretary, had spent more time scrutinising the original claims. It says the episode should have been investigated by Sir Alex Allan, Mr Cameron's adviser on the Ministerial Code.

"The events leading to the resignation of the Government Chief Whip again demonstrate that the Cabinet Secretary is not the appropriate person to investigate allegations of ministerial misconduct," the MPs say.

"His role is limited; there is already intense pressure on his time; and his role as investigator may conflict with his primary role, which is to support the daily work of the Prime Minister and the Government as a whole."

The MPs bemoan Sir Jeremy's failure to resolve the "discrepancies" between the police record and Mr Mitchell's insistence he did not use the politically-toxic term "pleb" or most of the other words attributed to him.

They say it is "surprising" he did not try to check the details contained in the police log or the account given by the officer involved in the incident to another Downing Street official.

The MPs note that Sir Jeremy watched the CCTV footage and concluded there were inconsistencies between the film and the account in the email which appeared to verify the police record. But he decided not to pursue it because, as he told the committee, "we decided on balance to let matters rest as they were" and he had only been given a "very limited remit".

The MPs say: "Regardless of what the Prime Minister had or had not asked him to do, on establishing there were unanswered questions about the incident, Sir Jeremy should have advised that these questions required further investigation."

Bernard Jenkin, the committee's chairman, said: "Given time, attention and with his relevant experience, Sir Alex might well have uncovered the truth. Sir Jeremy told us he had never assessed CCTV evidence before."

Last night a Downing Street spokesman said: "The Prime Minister accepts responsibility for all these issues. Jeremy has his full backing."

Mr Mitchell admits swearing once under his breath in the presence of officers, but denies aiming his words at them. He claims he has become the victim of a conspiracy by officers.

Sir Jeremy conceded, when he appeared before the committee, that he had been aware that Mr Mitchell could have been the victim of a "gigantic conspiracy".

But despite his concerns the Cabinet Secretary did not raise the matter either with the Metropolitan Police Commissioner or the Independent Police Complaints Commission.