The Cabinet minister at the centre of the "Plebgate" row was "stitched up" by police, an alleged whistleblower was reported to have said.
The Sun is being sued by Mitchell over its front-page account of an incident at the gates to 10 Downing Street. It is defending the claim under the Reynolds responsible journalism defence.
A police log of the incident was published in the Daily Telegraph.
The Sunday Times reported yesterday that a senior officer described how evidence was doctored against Andrew Mitchell, the former chief whip who resigned from Cabinet over the accusation that he launched a foul-mouthed rant at officers guarding Downing Street as he asked to cycle through the main gates in September last year.
The Sunday Times reports on what it says is an account by a senior officer of the alleged conspiracy against Mr Mitchell.
The whistleblower stated: "On the 18th September, 2012 Mr Mitchell had also insisted on being let out through the main gate. Following this [officer X] said to the other officers: 'Right, we can stitch him up'."
The whistleblower then went on to state that the word "plebs" was added by the named officer to the original police log of the conversation that Mitchell was said to have had with the officer who was guarding the gates on 19 September, the newspaper added.
Mitchell has always denied using this "toxic" phrase.
A Metropolitan Police spokesman said: "This has been a lengthy and complex investigation involving the supervision of the Independent Police Complaints Commission.
"Many lines of inquiry have been investigated and the final file is now with the Crown Prosecution Service who are considering all the evidence."