The Metropolitan Police vowed to get to the truth of claims a police officer, posing as a member of the public, falsely claimed to have seen a confrontation outside Downing Street between the former chief whip and police officers on duty.
While Mr Mitchell admitted swearing at the police, who refused to allow him to cycle through the main gates, he vehemently denied calling the officers plebs and morons.
The row ultimately led to him resigning from the Cabinet.
In the House of Commons, David Cameron told MPs: "A police officer posing as a member of the public and sending an email potentially to blacken the name of a Cabinet minister is a very serious issue and does need to be seriously investigated."
The Prime Minister said Scotland Yard was conducting an inquiry and "we should allow them to get to the truth".
In a statement, the Met admitted that, if there was evidence, its inquiries would consider a police conspiracy.
It said: "The allegation that a serving police officer fabricated evidence is extremely serious.
"It goes to the very heart of the public's trust in the police service."
Scotland Yard pointed out how a serving officer had been arrested at the weekend and the investigation was now being supervised by the Independent Police Complaints Commission.
Met Commissioner Bernard Hogan-Howe had said earlier this week the investigation would be over before Christmas, but yesterday the force made clear: "The investigation will not be short."
Released CCTV footage of the incident in September – made public by Mr Mitchell who received it under the Data Protection Act – raised doubts about the on duty police officers' version of events.
Firstly, questions were raised as to whether he could have said the remarks attributed to him in the time it took to wheel his cycle to the Downing Street side entrance and, secondly, the crowd of people alleged to have witnessed the incident was clearly not there in the footage.
At Westminster, Tory colleagues insisted the former chief whip should be reinstated.
David Davis, the former shadow home secretary, said his colleague had been the victim of a serious injustice and should be returned to high office.
Meantime, doubts were raised about the thoroughness of the earlier probe by Sir Jeremy Heywood, the Cabinet Secretary, who viewed the corroborative email that condemned the ex-chief whip and the CCTV footage of the alleged incident in Downing Street.
In a letter to Mr Cameron, Angus MacNeil, the SNP MP for the Western Isles, asked if there was any concern that Sir Jeremy had produced an "inconclusive report".
Mr Davis was also critical. He said: "All the investigations did not go to the heart. Look at the CCTV. See whether it is consistent with the allegations made. It plainly wasn't."
The Prime Minister's spokesman said the conclusion of the Cabinet Secretary's report was "that one could not be confident the evidence in the emails was reliable, including on the basis of a number of inconsistencies with the CCTV footage".
The corroborative email, published last night, turned out to be virtually identical in its content to the leaked police log.
But Mr Cameron's spokesman would not confirm whether Sir Jeremy had seen the log because, he stressed, it was now part of the official Met probe.
Lord Reid, the former Labour home secretary, said the row could "easily out of a relatively small event become once again a huge festering sore in the relationship between the Government and the police". He urged all sides to temper their language.