Senior Tory MP David Davis said the email, which reveals complaints about Mr Mitchell's apparent insistence on cycling through the Downing Street gates, was written to set up circumstances which would lead to the confrontation that sparked the scandal.
The email, obtained by the Times, was written by a constable seeking advice from his superiors on how to handle Mr Mitchell's demands. The officer was said to have complained that Mr Mitchell "keeps requesting to leave Downing Street via the main vehicle gates" and was "adamant he WAS GOING THROUGH THOSE GATES".
It was written on September 19, the night before the plebgate incident, and suggests that Mr Mitchell had previous disagreements with officers over the issue.
Mr Davis claimed the email was written four hours after an incident involving Mr Mitchell which showed it was designed to set up a future public confrontation
He said the email "undermines the police case" and backs up claims made by an anonymous whistleblower that the dispute was premeditated.
Mr Davis said most people would have asked their seniors to have a conversation with Mr Mitchell about the issue but the email instead ensured that those seniors would support police officers involved in any dispute.
Former Tory leadership candidate Mr Davis also bemoaned the "incredibly spun" confidential email being leaked and revealed he had asked Metropolitan Police Commissioner Sir Bernard Hogan-Howe for a full inquiry into the leak.
In a Commons debate on the Police Federation, Mr Davis said: "I do not want to widen this into a rerun of the Mitchell case, but let me say a couple of things about this.
"The House knows full well that I didn't really approve of the Leveson process and I'm a believer in a free press and very strongly so.
"But even I am astonished that after Leveson we see yet again a police force leaking a confidential document which actually the victim in this, Mr Mitchell, has not had access to, leaking it in an incredibly spun way.
"First off, I expect the Metropolitan Police Commissioner to have a proper leak inquiry into that and I've told him that this morning.
"The second point I'd make is the astonishing interpretation put upon the leak that was given.
"Because what this leak showed was that an officer, four hours after stopping Mr Mitchell, or attempting to stop Mr Mitchell going through the main gates of Downing Street - four hours afterwards, so not in a panic, not in a rush but in a premeditated way - calling up or writing to his seniors, not to say 'we have a security issue here, would somebody please have a conversation with Mr Mitchell to make sure that he understands that we can't let him through' - which would be the proper thing, which I think any of us would have done - but to set up a circumstance where it would be resolved by a public confrontation at the front gate, ensuring that his seniors support him in doing that which if anything actually reinforces the story we were told by an anonymous whistleblower that this was a premeditated action.
"So I think today's press coverage was not a very good reflection of the police in two ways - it undermined their main case but also it is something they simply should not have done under these circumstances."
Mr Mitchell was forced to resign last year after it was reported that he had repeatedly sworn at officers and called them "plebs" after they insisted he wheeled his bicycle through a side gate - a claim he has always denied.
Evidence subsequently emerged casting doubt on the police version and last week a serving officer was jailed for fabricating an account of the incident.
Chairman of the Home Affairs Select Committee Keith Vaz said he felt Mr Mitchell and his family had gone through a "terrible ordeal".
"I believe he has been vindicated in what has happened to him because of the fact that 11 of those involved in the case have now become the subject of misconduct hearings and one has gone to prison," Mr Vaz said.
"For those of us who have been around for a long time, you think if this could happen to a serving Cabinet minister, what hope for one of our constituents?
"I say this about what he has done - he has done the House and the public a great service from a position of power that he holds as an elected MP, quite different to someone else perhaps in Leicester or another part of this country.
"It is important a line is drawn and people move in for the sake of himself and his family and, indeed, the reputation of the police as a whole."
Mr Vaz said his committee would look into the impact of the Normington report (on Police Federation reform), adding he wanted to focus on money held by regional branches of the the federation.
He said further concerns were highlighted by the apparent lack of central awareness of the numbers and identities of federation members.
Labour MP David Lammy (Tottenham) said he enjoyed spending time with police officers in London and beyond through a parliamentary scheme.
He told MPs: "Overwhelmingly it has confirmed my childhood belief that began at the age of about nine, when I said to my parents, 'I want to be a police officer', that the men and women who serve in our police forces across this country do a fantastic job."
But he said the events surrounding Mr Mitchell had demonstrated problems in some parts of the police that urgently needed to have light shone on them.
He said: "All of us have had the privilege to travel to countries where corruption is endemic in the police force ... we all understand in a growing democracy, and certainly one such as ours, how we treat the most vulnerable, how we treat the areas of our life where light often doesn't shine, is an indication of our democracy itself.
"Because it is the job of the police to deal with, fortunately for our country, a small criminal minority ... the light has often not shone and certain practices have grown up.
"Many of us have watched in this country as cases involving minorities have often been overlooked and the truth is that there are many cases ... where there have been concerns about the federation and about a closed shop particularly in relation to getting at the truth.
"When you see three officers so blatantly tell mistruths, so blatantly refuse to apologise about a Cabinet minister in a country such as this, that must tell you something about a culture of impunity that has become endemic in the system.
"It must say something about the necessary reform that must now come."
Mr Mitchell was sat in the Commons listening to the debate, although he did not make a speech.
Conservative Michael Ellis (Northampton North), a barrister in criminal law for 15 years, said public trust in police was at its lowest ever level, adding: "And in large measure it is due to the disgraceful misconduct of the leadership and previous leadership within the national Police Federation."
He said regional branches of the federation did a good job, but the "egregious examples" needed to be looked at and addressed before they cause even greater damage to the country's reputation.
Turning to Mr Mitchell's case, Mr Ellis said an apology was still due.
He said: "If what happened to (Mr Mitchell) happened in those circumstances, in such a location, to such a person, what hope is there for one of my constituents or any of our constituents without that position, without that authority, without those resources to defend themselves?"
The Home Affairs Select Committee member said he agreed with Mr Vaz that Mr Mitchell had been "entirely vindicated".
Mr Ellis said: "(Mr Mitchell) has already received an apology and rightly so from several chief constables, I understand.
"But the fact there are several police officers facing internal misconduct or gross misconduct matters and one has gone to prison, I am appalled, as I know the House will be, that the federation is funding litigation even now, which seeks to keep this alive."
Mr Vaz recalled that officers from West Mercia, Warwickshire and West Midlands had a chance to draw a line under Mr Mitchell's case when they appeared before the Home Affairs Select Committee.
He asked Mr Ellis: "Even at this late stage do you not agree this is something that could be done in order to bring that whole sorry episode to a conclusion?"
Mr Ellis replied: "The fact is an apology is still due and you are quite right that their conduct and their appearance before the committee... was an embarrassment to the Police Federation."
Conservative backbencher Jacob Rees-Mogg (North East Somerset) expressed concern about the findings of the Normington report, which suggested a level of police corruption normally seen in "third world countries".
Describing the report as "stunning" and "enormously damning", he told MPs: "But what concerns me most is the constitutional aspect of this, because we know that the Police Federation conspired, lied and leaked to remove a Cabinet Minister from office."
He said all the evidence pointed to the police "conspiring to bring down a Cabinet Minister".
He added: "Now that is what happens in third world countries, where the democratic rights of people are overtaken by the forces of law and order who intervene to have the type of government they want, rather than the type of government that the people want.
"It is such a dangerous position to have got into, where a body that has these particular protections, this place in the state, is able to abuse them to undermine the very constitution that gives them those powers."